Personalizing Image Search Results on Flickr
Kristina Lerman, Anon Plangprasopchok and Chio Wong
University of Southern California
Information Sciences Institute
4676 Admiralty Way
Marina del Rey, California 90292
{lerman,plangpra,chiowong}@isi.edu
Abstract
The social media site Flickr allows users to upload their pho-
tos, annotate them with tags, submit them to groups, and also
to form social networks by adding other users as contacts.
Flickr offers multiple ways of browsing or searching it. One
option is tag search, which returns all images tagged with a
specific keyword. If the keyword is ambiguous, e.g., “beetle”
could mean an insect or a car, tag search results will include
many images that are not relevant to the sense the user had in
mind when executing the query. We claim that users express
their photography interests through the metadata they add in
the form of contacts and image annotations. We show how
to exploit this metadata to personalize search results for the
user, thereby improving search performance. First, we show
that we can significantly improve search precision by filtering
tag search results by user’s contacts or a larger social network
that includes those contact’s contacts. Secondly, we describe
a probabilistic model that takes advantage of tag information
to discover latent topics contained in the search results. The
users’ interests can similarly be described by the tags they
used for annotating their images. The latent topics found by
the model are then used to personalize search results by find-
ing images on topics that are of interest to the user.
Introduction
The photosharing site Flickr is one of the earliest and more
popular examples of the new generation of Web sites, la-
beled social media, whose content is primarily user-driven.
Other examples of social media include: blogs (personal
online journals that allow users to share thoughts and re-
ceive feedback on them), Wikipedia (a collectively writ-
ten and edited online encyclopedia), and Del.icio.us and
Digg (Web sites that allow users to share, discuss, and rank
Web pages, and news stories respectively). The rise of so-
cial media underscores a transformation of the Web as fun-
damental as its birth. Rather than simply searching for,
and passively consuming, information, users are collabora-
tively creating, evaluating, and distributing information. In
the near future, new information-processing applications en-
abled by social media will include tools for personalized in-
formation discovery, applications that exploit the “wisdom
of crowds” (e.g., emergent semantics and collaborative in-
Copyright c© 2018, American Association for Artificial Intelli-
gence (www.aaai.org). All rights reserved.
formation evaluation), deeper analysis of community struc-
ture to identify trends and experts, and many others still dif-
ficult to imagine.
Social media sites share four characteristics: (1) Users
create or contribute content in a variety of media types;
(2) Users annotate content with tags; (3) Users evaluate con-
tent, either actively by voting or passively by using content;
and (4) Users create social networks by designating other
users with similar interests as contacts or friends. In the pro-
cess of using these sites, users are adding rich metadata in
the form of social networks, annotations and ratings. Avail-
ability of large quantities of this metadata will lead to the
development of new algorithms to solve a variety of infor-
mation processing problems, from new recommendation to
improved information discovery algorithms.
In this paper we show how user-added metadata on Flickr
can be used to improve image search results. We claim that
users express their photography interests on Flickr, among
other ways, by adding photographers whose work they ad-
mire to their social network and through the tags they use
to annotate their own images. We show how to exploit this
information to personalize search results to the individual
user.
The rest of the paper is organized as follows. First, we
describe tagging and why it can be viewed as a useful ex-
pression of user’s interests, as well as some of the challenges
that arise when working with tags. In Section “Anatomy of
Flickr” we describe Flickr and its functionality in greater de-
tails, including its tag search capability. In Section “Data
collections” we describe the data sets we have collected
from Flickr, including image search results and user infor-
mation. In Sections “Personalizing by contacts” and “Per-
sonalizing by tags” we present the two approaches to per-
sonalize search results for an individual user by filtering by
contacts and filtering by tags respectively. We evaluate the
performance of each method on our Flickr data sets. We
conclude by discussing results and future work.
Tagging for organizing images
Tags are keyword-based metadata associated with some con-
tent. Tagging was introduced as a means for users to orga-
nize their own content in order to facilitate searching and
browsing for relevant information. It was popularized by
http://arxiv.org/abs/0704.1676v1
the social bookmarking site Delicious1, which allowed users
to add descriptive tags to their favorite Web sites. In re-
cent years, tagging has been adopted by many other so-
cial media sites to enable users to tag blogs (Technorati),
images (Flickr), music (Last.fm), scientific papers (CiteU-
Like), videos (YouTube), etc.
The distinguishing feature of tagging systems is that they
use an uncontrolled vocabulary. This is in marked contrast
to previous attempts to organize information via formal tax-
onomies and classification systems. A formal classification
system, e.g., Linnaean classification of living things, puts an
object in a unique place within a hierarchy. Thus, a tiger
(Panthera tigris) is a carnivorous mammal that belongs to
the genus Panthera, which also includes large cats, such as
lions and leopards. Tiger is also part of the felidae family,
which includes small cats, such as the familiar house cat of
the genus Felis.
Tagging is a non-hierarchical and non-exclusive cat-
egorization, meaning that a user can choose to high-
light any one of the tagged object’s facets or proper-
ties. Adapting the example from Golder and Huber-
man (Golder and Huberman 2005), suppose a user takes an
image of a Siberian tiger. Most likely, the user is not famil-
iar with the formal name of the species (P. tigris altaica) and
will tag it with the keyword “tiger.” Depending on his needs
or mood, the user may even tag is with more general or spe-
cific terms, such as “animal,” “mammal” or “Siberian.” The
user may also note that the image was taken at the “zoo”
and that he used his “telephoto” lens to get the shot. Rather
than forcing the image into a hierarchy or multiple hierar-
chies based on the equipment used to take the photo, the
place where the image was taken, type of animal depicted,
or even the animal’s provenance, tagging system allows the
user to locate the image by any of its properties by filtering
the entire image set on any of the tags. Thus, searching on
the tag “tiger” will return all the images of tigers the user has
taken, including Siberian and Bengal tigers, while searching
on “Siberian” will return the images of Siberian animals,
people or artifacts the user has photographed. Filtering on
both “Siberian” and “tiger” tags will return the intersection
of the images tagged with those keywords, in other words,
the images of Siberian tigers.
As Golder and Huberman point out, tagging systems are
vulnerable to problems that arise when users try to attach
semantics to objects through keywords. These problems are
exacerbated in social media where users may use different
tagging conventions, but still want to take advantage of the
others’ tagging activities. The first problem is of homonymy,
where the same tag may have different meanings. For exam-
ple, the “tiger” tag could be applied to the mammal or to
Apple computer’s operating system. Searching on the tag
“tiger” will return many images unrelated the carnivorous
mammals, requiring the user to sift through possibly a large
amount of irrelevant content. Another problem related to
homonymy is that of polysemy, which arises when a word
has multiple related meanings, such as “apple” to mean the
company or any of its products. Another problem is that
1http://del.icio.us
of synonymy, or multiple words having the same or related
meaning, for example, “baby” and “infant.” The problem
here is that if the user wants all images of young children
in their first year of life, searching on the tag “baby” may
not return all relevant images, since other users may have
tagged similar photographs with “infant.” Of course, plurals
(“tigers” vs ”tiger”) and many other tagging idiosyncrasies
(”myson” vs “son”) may also confound a tagging system.
Golder and Huberman identify yet another problem that
arises when using tags for categorization — that of the “ba-
sic level.” A given item can be described by terms along
a spectrum of specificity, ranging from specific to general.
A Siberian tiger can be described as a “tiger,” but also as
a “mammal” and “animal.” The basic level is the category
people choose for an object when communicating to others
about it. Thus, for most people, the basic level for canines
is “dog,” not the more general “animal” or the more specific
“beagle.” However, what constitutes the basic level varies
between individuals, and to a large extent depends on the
degree of expertise. To a dog expert, the basic level may be
the more specific “beagle” or “poodle,” rather than “dog.”
The basic level problem arises when different users choose
to describe the item at different levels of specificity. For ex-
ample, a dog expert tags an image of a beagle as “beagle,”
whereas the average user may tag a similar image as “dog.”
Unless the user is aware of the basic level variation and sup-
plies more specific (and more general) keywords during tag
search, he may miss a large number of relevant images.
Despite these problems, tagging is a light weight, flexi-
ble categorization system. The growing number of tagged
images provides evidence that users are adopting tag-
ging on Flickr (Marlow et al. 2006). There is specula-
tion (Mika 2005) that collective tagging will lead to a com-
mon informal classification system, dubbed a “folksonomy,”
that will be used to organize all information from all users.
Developing value-added systems on top of tags, e.g., which
allow users to better browse or search for relevant items, will
only accelerate wider acceptance of tagging.
Anatomy of Flickr
Flickr consists of a collection of interlinked user, photo, tag
and group pages. A typical Flickr photo page is shown in
Figure 1. It provides a variety of information about the im-
age: who uploaded it and when, what groups it has been sub-
mitted to, its tags, who commented on the image and when,
how many times the image was viewed or bookmarked as a
“favorite.” Clicking on a user’s name brings up that user’s
photo stream, which shows the latest photos she has up-
loaded, the images she marked as “favorite,” and her profile,
which gives information about the user, including a list of
her contacts and groups she belong to. Clicking on the tag
shows user’s images that have been tagged with this key-
word, or all public images that have been similarly tagged.
Finally, the group link brings up the group’s page, which
shows the photo group, group membership, popular tags,
discussions and other information about the group.
Figure 1: A typical photo page on Flickr
Groups Flickr allows users to create special interest
groups on any imaginable topic. There are groups for
showcasing exceptional images, group for images of circles
within a square, groups for closeups of flowers, for the color
red (and every other color and shade), groups for rating sub-
mitted images, or those used solely to generate comments.
Some groups are even set up as games, such as The Infinite
Flickr, where the rule is that a user post an image of her-
self looking at the screen showing the last image (of a user
looking at the screen showing next to last image, etc).
There is redundancy and duplication in groups. For exam-
ple, groups for child photography include Children’s Por-
traits, Kidpix, Flickr’s Cutest Kids, Kids in Action, Tod-
dlers, etc. A user chooses one, or usually several, groups to
which to submit an image. We believe that group names can
be viewed as a kind of publicly agreed upon tags.
Contacts Flickr allows users to designate others as friends
or contacts and makes it easy to track their activities. A
single click on the “Contacts” hyperlink shows the user the
latest images from his or her contacts. Tracking activities of
friends is a common feature of many social media sites and
is one of their major draws.
Interestingness Flickr uses the “interestingness” criterion
to evaluate the quality of the image. Although the algorithm
that is used to compute this is kept secret to prevent gaming
the system, certain metrics are taken into account: “where
the clickthroughs are coming from; who comments on it and
when; who marks it as a favorite; its tags and many more
things which are constantly changing.”2
Browsing and searching
Flickr offers the user a number of browsing and searching
methods. One can browse by popular tags, through the
groups directory, through the Explore page and the calen-
dar interface, which provides access to the 500 most “inter-
esting” images on any given day. A user can also browse
geotagged images through the recently introduced map in-
terface. Finally, Flickr allows for social browsing through
the “Contacts” interface that shows in one place the recent
images uploaded by the user’s designated contacts.
Flickr allows searching for photos using full text or tag
search. A user can restrict the search to all public photos,
his or her own photos, photos she marked as her favorite, or
photos from a specific contact. The advanced search inter-
face currently allows further filtering by content type, date
and camera.
Search results are by default displayed in reverse chrono-
logical order of being uploaded, with the most recent images
on top. Another available option is to display images by their
“interestingness” value, with the most “interesting” images
on top.
2http://flickr.com/explore/interesting/
Personalizing search results
Suppose a user is interested in wildlife photography and
wants to see images of tigers on Flickr. The user can search
for all public images tagged with the keyword “tiger.” As
of March 2007, such a search returns over 55, 500 results.
When images are arranged by their “interestingness,” the
first page of results contains many images of tigers, but
also of a tiger shark, cats, butterfly and a fish. Subsequent
pages of search results show, in addition to tigers, children in
striped suits, flowers (tiger lily), more cats, Mac OS X (tiger)
screenshots, golfing pictures (Tiger Woods), etc. In other
words, results include many false positives, images that are
irrelevant to what the user had in mind when executing the
search.
We assume that when the search term is ambiguous, the
sense that the user has in mind is related to her interests. For
example, when a child photographer is searching for pictures
of a “newborn,” she is most likely interested in photographs
of human babies, not kittens, puppies, or ducklings. Simi-
larly, a nature photographer specializing in macro photogra-
phy is likely to be interested in insects when searching on
the keyword “beetle,” not a Volkswagen car. Users express
their photography preferences and interests in a number of
ways on Flickr. They express them through their contacts
(photographers they choose to watch), through the images
they upload to Flickr, through the tags they add to these im-
ages, through the groups they join, and through the images
of other photographers they mark as their favorite. In this pa-
per we show that we can personalize results of tag search by
exploiting information about user’s preferences. In the sec-
tions below, we describe two search personalization meth-
ods: one that relies on user-created tags and one that exploits
user’s contacts. We show that both methods improve search
performance by reducing the number of false positives, or
irrelevant results, returned to the user.
Data collections
To show how user-created metadata can be used to personal-
ize results of tag search, we retrieved a variety of data from
Flickr using their public API.
Data sets
We collected images by performing a single keyword tag
search of all public images on Flickr. We specified that the
returned images are ordered by their “interestingness” value,
with most interesting images first. We retrieved the links to
the top 4500 images for each of the following search terms:
tiger possible senses include (a) big cat ( e.g., Asian tiger),
(b) shark (Tiger shark), (c) flower (Tiger Lily), (d) golfing
(Tiger Woods), etc.
newborn possible senses include (a) a human baby, (b) kit-
ten, (c) puppy, (d) duckling, (e) foal, etc.
beetle possible senses include (a) a type of insect and (b)
Volkswagen car model
For each image in the set, we used Flickr’s API to retrieve
the name of the user who posted the image (image owner),
and all the image’s tags and groups.
query relevant not relevant precision
newborn 412 83 0.82
tiger 337 156 0.67
beetle 232 268 0.46
Table 1: Relevance results for the top 500 images retrieved
by tag search
Users
Our objective is to personalize tag search results; therefore,
to evaluate our approach, we need to have users to whose
interests the search results are being tailored. We identi-
fied four users who are interested in the first sense of each
search term. For the newborn data set, those users were one
of the authors of the paper and three other contacts within
that user’s social network who were known to be interested
in child photography. For the other datasets, the users were
chosen from among the photographers whose images were
returned by the tag search. We studied each user’s profile
to confirm that the user was interested in that sense of the
search term. We specifically looked at group membership
and user’s tags. Thus, for the tiger data set, groups that
pointed to the user’s interest in P. tigris were Big Cats, Zoo,
The Wildlife Photography, etc. In addition to group mem-
bership, tags that pointed to user’s interest in a topic, e.g., for
the beetle data set, we assumed that users who used tags na-
ture and macro were interested in insects rather than cars.
Likewise, for the newborn data set, users who had uploaded
images they tagged with baby and child were probably in-
terested in human newborns.
For each of the twelve users, we collected the names of
their contacts, or Level 1 contacts. For each of these con-
tacts, we also retrieved the list of their contacts. These are
called Level 2 contacts. In addition to contacts, we also re-
trieved the list of all the tags, and their frequencies, that the
users had used to annotate their images. In addition to all
tags, we also extracted a list of related tags for each user.
These are the tags that appear together with the tag used as
the search term in the user’s photos. In other words, suppose
a user, who is a child photographer, had used tags such as
“baby”, “child”, “newborn”, and “portrait” in her own im-
ages. Tags related to newborn are all the tags that co-occur
with the “newborn” tag in the user’s own images. This in-
formation was also extracted via Flickr’s API.
Search results
We manually evaluated the top 500 images in each data set
and marked each as relevant if it was related to the first sense
of the search term listed above, not relevant or undecided, if
the evaluator could not understand the image well enough to
judge its relevance.
In Table 1, we report the precision of the search within the
500 labeled images, as judged from the point of view of the
searching users. Precision is defined as the ratio of relevant
images within the result set over the 500 retrieved images.
Precision of tag search on these sample queries is not very
high due to the presence of false positives — images not rel-
evant to the sense of the search term the user had in mind.
In the sections below we show how to improve search per-
formance by taking into consideration supplementary infor-
mation about user’s interests provided by her contacts and
tags.
Personalizing by contacts
Flickr encourages users to designate others as contacts by
making is easy to view the latest images submitted by them
through the “Contacts” interface. Users add contacts for a
variety of reasons, including keeping in touch with friends
and family, as well as to track photographers whose work
is of interest to them. We claim that the latter reason is the
most dominant of the reasons. Therefore, we view user’s
contacts as an expression of the user’s interests. In this sec-
tion we show that we can improve tag search results by filter-
ing through the user’s contacts. To personalize search results
for a particular user, we simply restrict the images returned
by the tag search to those created by the user’s contacts.
Table 2 shows how many of the 500 images in each data
set came from a user’s contacts. The column labeled “# L1”
gives the number of user’s Level 1 contacts. The follow-
ing columns show how many of the images were marked as
relevant or not relevant by the filtering method, as well as
precision and recall relative to the 500 images in each data
set. Recall measures the fraction of relevant retrieved im-
ages relative to all relevant images within the data set. The
last column “improv” shows percent improvement in preci-
sion over the plain (unfiltered) tag search.
As Table 2 shows, filtering by contacts improves the pre-
cision of tag search for most users anywhere from 22% to
over 100% when compared to plain search results in Ta-
ble 1. The best performance is attained for users within the
newborn set, with a large number of relevant images cor-
rectly identified as being relevant, and no irrelevant images
admitted into the result set. The tiger set shows an average
precision gain of 42% over four users, while the beetle set
shows an 85% gain.
Increase in precision is achieved by reducing the number
of false positives, or irrelevant images that are marked as rel-
evant by the search method. Unfortunately, this gain comes
at the expense of recall: many relevant images are missed
by this filtering method. In order to increase recall, we en-
large the contacts set by considering two levels of contacts:
user’s contacts (Level 1) and her contacts’ contacts (Level
2). The motivation for this is that if the contact relation-
ship expresses common interests among users, user’s inter-
ests will also be similar to those of her contacts’ contacts.
The second half of Table 2 shows the performance of
filtering the search results by the combined set of user’s
Level 1 and Level 2 contacts. This method identifies many
more relevant images, although it also admits more irrele-
vant images, thereby decreasing precision. This method still
shows precision improvement over plain search, with pre-
cision gain of 9%, 16% and 11% respectively for the three
data sets.
Personalizing by tags
In addition to creating lists of contacts, users express their
photography interests through the images they post on
Flickr. We cannot yet automatically understand the content
of images. Instead, we turn to the metadata added by the
user to the image to provide a description of the image. The
metadata comes in a variety of forms: image title, descrip-
tion, comments left by other users, tags the image owner
added to it, as well as the groups to which she submitted the
image. As we described in the paper, tags are useful im-
age descriptors, since they are used to categorize the image.
Similarly, group names can be viewed as public tags that a
community of users have agreed on. Submitting an image to
a group is, therefore, equivalent to tagging it with a public
In the section below we describe a probabilistic model
that takes advantage of the images’ tag and group informa-
tion to discover latent topics in each search set. The users’
interests can similarly be described by collections of tags
they had used to annotate their own images. The latent top-
ics found by the model can be used to personalize search
results by finding images on topics that are of interest to a
particular user.
Model definition
We need to consider four types of entities in the model: a
set of users U = {u1, ..., un}, a set of images or photos
I = {i1, ..., im}, a set of tags T = {t1, ..., to}, and a set
of groups G = {g1, ..., gp}. A photo ix posted by owner
ux is described by a set of tags {tx1, tx2, ...} and submitted
to several groups {gx1, gx2, ...}. The post could be viewed
as a tuple < ix, ux, {tx1, tx2, ...}, {gx1, gx2, ...} >. We as-
sume that there are n users, m posted photos and p groups
in Flickr. Meanwhile, the vocabulary size of tags is q. In
order to filter images retrieved by Flickr in response to tag
search and personalize them for a user u, we compute the
conditional probability p(i|u), that describes the probability
that the photo i is relevant to u based on her interests. Im-
ages with high enough p(i|u) are then presented to the user
as relevant images.
As mentioned earlier, users choose tags from an uncon-
trolled vocabulary according to their styles and interests.
Images of the same subject could be tagged with different
keywords although they have similar meaning. Meanwhile,
the same keyword could be used to tag images of different
subjects. In addition, a particular tag frequently used by one
user may have a different meaning to another user. Proba-
bilistic models offer a mechanism for addressing the issues
of synonymy, polysemy and tag sparseness that arise in tag-
ging systems.
We use a probabilistic topic
model (Rosen-Zvi et al. 2004) to model user’s image
posting behavior. As in a typical probabilistic topic model,
topics are hidden variables, representing knowledge cate-
gories. In our case, topics are equivalent to image owner’s
interests. The process of photo posting by a particular user
could be described as a stochastic process:
• User u decides to post a photo i.
user # L1 rel. not rel. Pr Re improv # L2+L2 rel. not rel. Pr Re improv
newborn
user1 719 232 0 1.00 0.56 22% 49,539 349 62 0.85 0.85 4%
user2 154 169 0 1.00 0.41 22% 10,970 317 37 0.9 0.77 10%
user3 174 147 0 1.00 0.36 22% 13,153 327 39 0.89 0.79 9%
user4 128 132 0 1.00 0.32 22% 8,439 310 29 0.91 0.75 11%
tiger
user5 63 11 1 0.92 0.03 37% 13,142 255 71 0.78 0.76 16%
user6 103 78 3 0.96 0.23 44% 14,425 266 83 0.76 0.79 13%
user7 62 65 1 0.98 0.19 47% 7,270 226 60 0.79 0.67 18%
user8 56 30 0 0.97 0.09 44% 7,073 240 63 0.79 0.71 18%
beetle
user9 445 18 1 0.95 0.08 106% 53,480 215 221 0.49 0.93 7%
user10 364 35 8 0.81 0.15 77% 41,568 208 217 0.49 0.90 7%
user11 783 78 25 0.75 0.34 65% 62,610 218 227 0.49 0.94 7%
user12 102 7 1 0.88 0.03 90% 14,324 163 152 0.52 0.70 13%
Table 2: Results of filtering tag search by user’s contacts. “# L1” denotes the number of Level 1 contacts and “# L1+L2” shows
the number of Level 1 and Level 2 contacts, with the succeeding columns displaying filtering results of that method: the number
of images marked relevant or not relevant, as well as precision and recall of the filtering method relative to the top 500 images.
The columns marked “improv” show improvement in precision over plain tag search results.
Figure 2: Graphical representation for model-based infor-
mation filtering. U , T , G and Z denote variables “User”,
“Tag”, “Group”, and “Topic” respectively. Nt represents a
number of tag occurrences for a one photo (by the photo
owner); D represents a number of all photos on Flickr.
Meanwhile, Ng denotes a number of groups for a particu-
lar photo.
• Based on user u’s interests and the subject of the photo, a
set of topics z are chosen.
• Tag t is then selected based on the set of topics chosen in
the previous state.
• In case that u decides to expose her photo to some groups,
a group g is then selected according to the chosen topics.
The process is depicted in a graphical form in Figure 2.
We do not treat the image i as a variable in the model but
view it as a co-occurrence of a user, a set of tags and a set of
groups. From the process described above, we can represent
the joint probability of user, tag and group for a particular
photo as
p(i) = p(ui, Ti, Gi)
= p(ui) ·
p(zk|ui)p(ti|z)
)ni(t)
p(zk|ui)p(gi|z)
)ni(g)
Note that it is straightforward to exclude photo’s group
information from the above equation simply by omitting the
terms relevant to g. nt and ng is a number of all possible tags
and groups respectively in the data set. Meanwhile, ni(t)
and ni(g) act as indicator functions: ni(t) = 1 if an image i
is tagged with tag t; otherwise, it is 0. Similarly, ni(g) = 1
if an image i is submitted to group g; otherwise, it is 0. k is
the predefined number of topics.
The joint probability of photos in the data set I is defined
p(I) =
p(im).
In order to estimate parameters p(z|ui), p(ti|z), and p(gi|z),
we define a log likelihood L, which measures how the esti-
mated parameters fit the observed data. According to the
EM algorithm (Dempster et al. 1977), L will be used as an
objective function to estimate all parameters. L is defined as
L(I) = log(p(I)).
In the expectation step (E-step), the joint probability of
the hidden variable Z given all observations is computed
from the following equations:
p(z|t, u) ∝ p(z|u) · p(t|z) (1)
p(z|g, u) ∝ p(z|u) · p(g|z). (2)
L cannot be maximized easily, since the summation over
the hidden variable Z appears inside the logarithm. We in-
stead maximize the expected complete data log-likelihood
over the hidden variable, E[Lc], which is defined as
E[Lc] =
log(p(u))
ni(t) ·
p(z|u, t) (log(p(z|u)· (t|z))
ni(g) ·
p(z|u, g) (log(p(z|g)· (g|z))
Since the term
log(p(ui)) is not related to parame-
ters and can be computed directly from the observed data,
we discard this term from the expected complete data log-
likelihood. With normalization constraints on all parame-
ters, Lagrange multipliers τ , ρ, ψ are added to the expected
log likelihood, yielding the following equation
H = E[Lc] +
p(t|z)
p(g|z)
p(z|u)
We maximize H with respect to p(t|zk), p(g|zk), and
p(zk|u), and then eliminate the Lagrange multipliers to ob-
tain the following equations for the maximization step:
p(t|z) ∝
ni(t) · p(z|t, u) (3)
p(g|z) ∝
ni(g) · p(z|g, u) (4)
p(zk|um) ∝
nm(t) · p(zk|um, t) (5)
nm(g) · p(zk|um, g)
The algorithm iterates between E and M step until the log
likelihood for all parameter values converge.
Model-based personalization
We can use the model developed in the previous section to
find the images i most relevant to the interests of a partic-
ular user u′. We do so by learning the parameters of the
model from the data and using these parameters to compute
the conditional probability p(i|u′). This probability can be
factorized as follows:
p(i|u′) =
p(ui, Ti, Gi|z) · p(z|u
′) , (6)
where ui is the owner of image i in the data set, and Ti and
Gi are, respectively, the set of all the tags and groups for the
image i.
The former term in Equation 6 can be factorized further
p(ui, Ti, Gi|z) ∝ p(Ti|z)· (Gi|z)· (z|ui) · p(ui)
p(ti|z)
p(gi|z)
· p(z|ui) · p(ui) .
We can use the learned parameters to compute this term di-
rectly.
We represent the interests of user u′ as an aggregate of the
tags that u′ had used in the past for tagging her own images.
This information is used to to approximate p(z|u′):
p(z|u′) ∝
n(t′ = t) · p(z|t)
where n(t′ = t) is a frequency (or weight) of tag t′ used
by u′. Here we view n(t′ = t) is proportional to p(t′|u′).
Note that we can use either all the tags u′ had applied to the
images in her photostream, or a subset of these tags, e.g.,
only those that co-occur with some tag in user’s images.
Evaluation
We trained the model separately on each data set of 4500
images. We fixed the number of topics at ten. We then eval-
uated our model-based personalization framework by using
the learned parameters and the information about the in-
terests of the selected users to compute p(i|u′) for the top
500 (manually labeled) images in the set. Information about
user’s interests was captured either by (1) all tags (and their
frequencies) that are used in all the images of the user’s pho-
tostream or (2) related tags that occurred in images that were
tagged with the search keyword (e.g., “newborn”) by the
user.
Computation of p(t|z) is central to the parameter estima-
tion process, and it tells us something about how strongly a
tag t contributes to a topic z. Table 3 shows the most prob-
able 25 tags for each topic for the tiger data set trained on
ten topics. Although the tag “tiger” dominates most topics,
we can discern different themes from the other tags that ap-
pear in each topic. Thus, topic z5 is obviously about domes-
tic cats, while topic z8 is about Apple computer products.
Meanwhile, topic z2 is about flowers and colors (“flower,”
“lily,” “yellow,” “pink,” “red”); topic z6 is about about
places (“losangeles,” “sandiego,” “lasvegas,” “stuttgard,”),
presumably because these places have zoos. Topic z7 con-
tains several variations of tiger’s scientific name, “panthera
tigris.” This method appears to identify related words well.
Topic z5, for example, gives synonyms “cat,” “kitty,” as well
as the more general term “pet” and the more specific terms
“kitten” and “tabby.” It even contains the Spanish version of
the word: “gatto.” In future work we plan to explore using
this method to categorize photos in a more abstract way. We
also note that related terms can be used to increase search
recall by providing additional keywords for queries.
Table 4 presents results of model-based personalization
for the case that uses information from all of user’s tags.
The model was trained with ten topics. Results are pre-
sented for different thresholds. The first two columns, for
example, report precision and recall for a high threshold that
z1 z2 z3 z4 z5
tiger tiger tiger tiger tiger
zoo specanimal cat thailand cat
animal animalkingdomelite kitty bengal animal
nature abigfave cute animals animals
animals flower kitten tigers zoo
wild butterfly cats canon bigcat
tijger macro orange d50 cats
wildlife yellow eyes tigertemple tigre
ilovenature swallowtail pet 20d animalplanet
cub lily tabby white tigers
siberiantiger green stripes nikon bigcats
blijdorp canon whiskers kanchanaburi whitetiger
london insect white detroit mammal
australia nature art life wildlife
portfolio pink feline michigan colorado
white red fur detroitzoo stripes
dierentuin flowers animal eos denver
toronto orange gatto temple sumatrantiger
stripes eastern pets park white
amurtiger usa black asia feline
nikonstunninggallery impressedbeauty paws ball mammals
s5600 tag2 furry marineworld sumatran
eyes specnature nose baseball exoticcats
sydney black teeth detroittigers exoticcat
cat streetart beautiful wild big
z6 z7 z8 z9 z10
tiger nationalzoo tiger tiger tiger
tigers tiger apple india lion
dczoo sumatrantiger mac canon dog
tigercub zoo osx wildlife shark
california nikon macintosh impressedbeauty nyc
lion washingtondc screenshot endangered cat
cat smithsonian macosx safari man
cc100 washington desktop wildanimals people
florida animals imac wild arizona
girl cat stevejobs tag1 rock
wilhelma bigcat dashboard tag3 beach
self tigris macbook park sand
lasvegas panthera powerbook taggedout sleeping
stuttgart bigcats os katze tree
me d70s 104 nature forest
baby pantheratigrissumatrae canon bravo puppy
tattoo dc x nikon bird
endangered sumatrae ipod asia portrait
illustration animal computer canonrebelxt marwell
?? 2005 ibook bandhavgarh boy
losangeles pantheratigris intel vienna fish
portrait nikond70 keyboard schnbrunn panther
sandiego d70 widget zebra teeth
lazoo 2006 wallpaper pantheratigris brooklyn
giraffe topv111 laptop d2x bahamas
Table 3: Top tags ordered by p(t—z) for the ten topic model of the “tiger” data set.
Pr Re Pr Re Pr Re Pr Re Pr Re
newborn
n=50 n=100 n=200 n=300 n=412*
user1 1.00 0.12 1.00 0.24 1.00 0.49 0.94 0.68 0.89 0.89
user2 1.00 0.12 1.00 0.24 1.00 0.49 0.92 0.67 0.87 0.87
user3 1.00 0.12 0.88 0.21 0.84 0.41 0.85 0.62 0.89 0.89
user4 1.00 0.12 0.99 0.24 1.00 0.48 0.94 0.69 0.89 0.89
tiger
n=50 n=100 n=200 n=300 n=337*
user5 0.94 0.14 0.90 0.27 0.82 0.48 0.80 0.71 0.79 0.79
user6 0.76 0.11 0.80 0.24 0.79 0.47 0.77 0.69 0.77 0.77
user7 0.94 0.14 0.90 0.27 0.82 0.48 0.80 0.71 0.79 0.79
user8 0.90 0.13 0.88 0.26 0.82 0.49 0.79 0.71 0.79 0.79
beetle
n=50 n=100 n=200 n=232* n=300
user9 1.00 0.22 0.99 0.43 0.77 0.66 0.70 0.70 0.66 0.85
user10 0.98 0.21 0.99 0.43 0.77 0.66 0.70 0.70 0.66 0.85
user11 0.98 0.21 0.93 0.40 0.50 0.43 0.51 0.51 0.50 0.65
user12 1.00 0.22 0.99 0.43 0.77 0.66 0.70 0.70 0.66 0.85
Table 4: Filtering results where a number of learned topics is 10, excluding group information, and user’s personal information
obtained from all tags she used for her photos. Asterisk denotes R-precision of the method, or precision of the first n results,
where n is the number of relevant results in the data set.
marks only the 50 most probable images as relevant. The re-
maining 450 images are marked as not relevant to the user.
Recall is low, because many relevant images are excluded
from the results for such a high threshold. As the thresh-
old is decreased (n = 100, n = 200, . . .), recall relative to
the 500 labeled images increases. Precision remains high in
all cases, and higher than precision of the plain tag search
reported in Table 1. In fact, most of the images in the top
100 results presented to the user are relevant to her query.
The column marked with the asterisk gives the R-precision
of the method, or precision of the first R results, where R is
the number of relevant results. The average R-precision of
this filtering method is 8%, 17% and 42% better than plain
search precision on our three data sets.
Performance results of the approach that uses related tags
instead of all tags are given in Table 5. We explored this di-
rection, because we believed it could help discriminate be-
tween different topics that interest a user. Suppose, a child
photographer is interested in nature photography as well as
child portraiture. The subset of tags he used for tagging his
“newborn” portraits will be different from the tags used for
tagging nature images. These tags could be used to differen-
tiate between newborn baby and newborn colt images. How-
ever, on the set of users selected for our study, using related
tags did not appear to improve results. This could be be-
cause the tags a particular user used together with, for ex-
ample, “beetle” do not overlap significantly with the rest of
the data set.
Including group information did not significantly improve
results (not presented in this manuscript). In fact, group in-
formation sometimes hurts the estimation rather than helps.
We believe that this is because our data sets (sorted by Flickr
according to image interestingness) are biased by the pres-
ence of general topic groups (e.g., Search the Best, Spec-
tacular Nature, Let’s Play Tag, etc.). We postulate that
group information would help estimate p(i|z) in cases where
the photo has few or no tags. Group information would help
filling in the missing data by using group name as another
tag. We also trained the model on the data with 15 topics,
but found no significant difference in results.
Previous research
Recommendation or personalization systems can be cate-
gorized into two main categories. One is collaborative fil-
tering (Breese et al. 1998) which exploits item ratings from
many users to recommend items to other like-minded users.
The other is content-based recommendation, which relies
on the contents of an item and user’s query, or other user
information, for prediction (Mooney and Roy 2000). Our
first approach, filtering by contacts, can be viewed as im-
plicit collaborative filtering, where the user–contact rela-
tionship is viewed as a preference indicator: it assumes
that the user likes all photos produced by her contacts. In
our previous work, we showed that users do indeed agree
with the recommendations made by contacts (Lerman 2007;
Lerman and Jones 2007). This is similar to the ideas imple-
mented by MovieTrust (Golbeck 2006), but unlike that sys-
tem, social media sites do not require users to rate their trust
in the contact.
Meanwhile, our second approach, filtering by tags (and
groups), shares some characteristics with both methods. It
is similar to collaborative filtering, since we use tags to rep-
resent agreement between users. It is also similar to content-
based recommendation, because we represent image content
by the tags and group names that have been assigned to it by
the user.
Our model-based filtering system is technically similar to,
but conceptually different from, probabilistic models pro-
Pr Re Pr Re Pr Re Pr Re Pr Re
newborn
n=50 n=100 n=200 n=300 n=412*
Pr Re Pr Re Pr Re Pr Re Pr Re
user1 0.8 0.10 0.78 0.19 0.79 0.38 0.77 0.56 0.79 0.79
user2 0.8 0.10 0.82 0.20 0.80 0.39 0.77 0.56 0.83 0.83
user3 0.98 0.12 0.88 0.21 0.84 0.41 0.80 0.58 0.85 0.85
user4 0.98 0.12 0.88 0.21 0.84 0.41 0.85 0.62 0.88 0.88
tiger
n=50 n=100 n=200 n=300 n=337*
user5 0.84 0.12 0.86 0.26 0.78 0.46 0.78 0.69 0.77 0.77
user6 0.72 0.11 0.79 0.23 0.78 0.46 0.76 0.68 0.76 0.76
user7 0.72 0.11 0.78 0.23 0.78 0.46 0.76 0.68 0.76 0.76
user8 0.9 0.13 0.82 0.24 0.80 0.47 0.78 0.69 0.78 0.78
beetle
n=50 n=100 n=200 n=232* n=300
user9 0.78 0.17 0.62 0.27 0.58 0.50 0.54 0.54 0.53 0.68
user10 0.98 0.21 0.88 0.38 0.77 0.66 0.72 0.72 0.65 0.84
user11 0.96 0.21 0.74 0.32 0.62 0.53 0.59 0.59 0.56 0.72
user12 0.98 0.21 0.99 0.43 0.77 0.66 0.70 0.70 0.66 0.85
Table 5: Filtering results where a number of learned topics is 10, excluding group information, and user’s personal information
obtained from all tags she used for her photos, which are tagged by the search term
posed by (Popescul et al. 2001). Both models are proba-
bilistic generative models that describe co-occurrences of
users and items of interest. In particular, the model assumes
a user generates her topics of interest; then the topics gen-
erate documents and words in those documents if the user
prefers those documents. In our model, we metaphorically
assume the photo owner generates her topics of interest. The
topics, in turn, generate tags that the owner used to annotate
her photo. However, unlike the previous work, we do not
treat photos as variables, as they do for documents. This is
because images are tagged only by their owners; meanwhile,
in their model, all users who are interested in a document
generate topics for that document.
Our model-based approach is almost identical to the
author-topic model(Rosen-Zvi et al. 2004). However, we
extend their framework to address (1) how to exploit photo’s
group information for personalized information filtering; (2)
how to approximate user’s topics of interest from partially
observed personal information (the tags the user used to de-
scribe her own images). For simplicity, we use the classi-
cal EM algorithm to train the model; meanwhile they use a
stochastic approximation approach due to the difficulty in-
volved in performing exact an inference for their generative
model.
Conclusions and future work
We presented two methods for personalizing results of im-
age search on Flickr. Both methods rely on the meta-
data users create through their everyday activities on Flickr,
namely user’s contacts and the tags they used for annotating
their images. We claim that this information captures user’s
tastes and preferences in photography and can be used to
personalize search results to the individual user. We showed
that both methods dramatically increase search precision.
We believe that increasing precision is an important goal for
personalization, because dealing with the information over-
load is the main issue facing users, and we can help users
by reducing the number of irrelevant results the user has to
examine (false positives). Having said that, our tag-based
approach can also be used to expand the search by suggest-
ing relevant related keywords (e.g., “pantheratigris,” “big-
cat” and ”cub” for the query tiger).
In addition to tags and contacts, there exists other meta-
data, favorites and comments, that can be used to aid infor-
mation personalization and discovery. In our future work
we plan to address the challenge of combing these heteroge-
neous sources of evidence within a single approach. We will
begin by combining contacts information with tags.
The probabilistic model needs to be explored further.
Right now, there is no principled way to pick the number
of latent topics that are contained in a data set. We also plan
to have a better mechanism for dealing with uninformative
tags and groups. We would like to automatically identify
general interest groups, such as the Let’s Play Tag group,
that do not help to discriminate between topics.
The approaches described here can be applied to other so-
cial media sites, such as Del.icio.us. We imagine that in
near future, all of Web will be rich with metadata, of the sort
described here, that will be used to personalize information
search and discovery to the individual user.
Acknowledgements
This research is based on work supported in part by the Na-
tional Science Foundation under Award Nos. IIS-0535182
and in part by DARPA under Contract No. NBCHD030010.
The U.S.Government is authorized to reproduce and dis-
tribute reports for Governmental purposes notwithstanding
any copyright annotation thereon. The views and conclu-
sions contained herein are those of the authors and should
not be interpreted as necessarily representing the official
policies or endorsements, either expressed or implied, of
any of the above organizations or any person connected with
them.
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	Introduction
	Tagging for organizing images
	Anatomy of Flickr
	Browsing and searching
	Personalizing search results
	Data collections
	Data sets
	Users
	Search results
	Personalizing by contacts
	Personalizing by tags
	Model definition
	Model-based personalization
	Evaluation
	Previous research
	Conclusions and future work

  The social media site Flickr allows users to upload their photos, annotate
them with tags, submit them to groups, and also to form social networks by
adding other users as contacts. Flickr offers multiple ways of browsing or
searching it. One option is tag search, which returns all images tagged with a
specific keyword. If the keyword is ambiguous, e.g., ``beetle'' could mean an
insect or a car, tag search results will include many images that are not
relevant to the sense the user had in mind when executing the query. We claim
that users express their photography interests through the metadata they add in
the form of contacts and image annotations. We show how to exploit this
metadata to personalize search results for the user, thereby improving search
performance. First, we show that we can significantly improve search precision
by filtering tag search results by user's contacts or a larger social network
that includes those contact's contacts. Secondly, we describe a probabilistic
model that takes advantage of tag information to discover latent topics
contained in the search results. The users' interests can similarly be
described by the tags they used for annotating their images. The latent topics
found by the model are then used to personalize search results by finding
images on topics that are of interest to the user.

Introduction
The photosharing site Flickr is one of the earliest and more
popular examples of the new generation of Web sites, la-
beled social media, whose content is primarily user-driven.
Other examples of social media include: blogs (personal
online journals that allow users to share thoughts and re-
ceive feedback on them), Wikipedia (a collectively writ-
ten and edited online encyclopedia), and Del.icio.us and
Digg (Web sites that allow users to share, discuss, and rank
Web pages, and news stories respectively). The rise of so-
cial media underscores a transformation of the Web as fun-
damental as its birth. Rather than simply searching for,
and passively consuming, information, users are collabora-
tively creating, evaluating, and distributing information. In
the near future, new information-processing applications en-
abled by social media will include tools for personalized in-
formation discovery, applications that exploit the “wisdom
of crowds” (e.g., emergent semantics and collaborative in-
Copyright c© 2018, American Association for Artificial Intelli-
gence (www.aaai.org). All rights reserved.
formation evaluation), deeper analysis of community struc-
ture to identify trends and experts, and many others still dif-
ficult to imagine.
Social media sites share four characteristics: (1) Users
create or contribute content in a variety of media types;
(2) Users annotate content with tags; (3) Users evaluate con-
tent, either actively by voting or passively by using content;
and (4) Users create social networks by designating other
users with similar interests as contacts or friends. In the pro-
cess of using these sites, users are adding rich metadata in
the form of social networks, annotations and ratings. Avail-
ability of large quantities of this metadata will lead to the
development of new algorithms to solve a variety of infor-
mation processing problems, from new recommendation to
improved information discovery algorithms.
In this paper we show how user-added metadata on Flickr
can be used to improve image search results. We claim that
users express their photography interests on Flickr, among
other ways, by adding photographers whose work they ad-
mire to their social network and through the tags they use
to annotate their own images. We show how to exploit this
information to personalize search results to the individual
user.
The rest of the paper is organized as follows. First, we
describe tagging and why it can be viewed as a useful ex-
pression of user’s interests, as well as some of the challenges
that arise when working with tags. In Section “Anatomy of
Flickr” we describe Flickr and its functionality in greater de-
tails, including its tag search capability. In Section “Data
collections” we describe the data sets we have collected
from Flickr, including image search results and user infor-
mation. In Sections “Personalizing by contacts” and “Per-
sonalizing by tags” we present the two approaches to per-
sonalize search results for an individual user by filtering by
contacts and filtering by tags respectively. We evaluate the
performance of each method on our Flickr data sets. We
conclude by discussing results and future work.
Tagging for organizing images
Tags are keyword-based metadata associated with some con-
tent. Tagging was introduced as a means for users to orga-
nize their own content in order to facilitate searching and
browsing for relevant information. It was popularized by
http://arxiv.org/abs/0704.1676v1
the social bookmarking site Delicious1, which allowed users
to add descriptive tags to their favorite Web sites. In re-
cent years, tagging has been adopted by many other so-
cial media sites to enable users to tag blogs (Technorati),
images (Flickr), music (Last.fm), scientific papers (CiteU-
Like), videos (YouTube), etc.
The distinguishing feature of tagging systems is that they
use an uncontrolled vocabulary. This is in marked contrast
to previous attempts to organize information via formal tax-
onomies and classification systems. A formal classification
system, e.g., Linnaean classification of living things, puts an
object in a unique place within a hierarchy. Thus, a tiger
(Panthera tigris) is a carnivorous mammal that belongs to
the genus Panthera, which also includes large cats, such as
lions and leopards. Tiger is also part of the felidae family,
which includes small cats, such as the familiar house cat of
the genus Felis.
Tagging is a non-hierarchical and non-exclusive cat-
egorization, meaning that a user can choose to high-
light any one of the tagged object’s facets or proper-
ties. Adapting the example from Golder and Huber-
man (Golder and Huberman 2005), suppose a user takes an
image of a Siberian tiger. Most likely, the user is not famil-
iar with the formal name of the species (P. tigris altaica) and
will tag it with the keyword “tiger.” Depending on his needs
or mood, the user may even tag is with more general or spe-
cific terms, such as “animal,” “mammal” or “Siberian.” The
user may also note that the image was taken at the “zoo”
and that he used his “telephoto” lens to get the shot. Rather
than forcing the image into a hierarchy or multiple hierar-
chies based on the equipment used to take the photo, the
place where the image was taken, type of animal depicted,
or even the animal’s provenance, tagging system allows the
user to locate the image by any of its properties by filtering
the entire image set on any of the tags. Thus, searching on
the tag “tiger” will return all the images of tigers the user has
taken, including Siberian and Bengal tigers, while searching
on “Siberian” will return the images of Siberian animals,
people or artifacts the user has photographed. Filtering on
both “Siberian” and “tiger” tags will return the intersection
of the images tagged with those keywords, in other words,
the images of Siberian tigers.
As Golder and Huberman point out, tagging systems are
vulnerable to problems that arise when users try to attach
semantics to objects through keywords. These problems are
exacerbated in social media where users may use different
tagging conventions, but still want to take advantage of the
others’ tagging activities. The first problem is of homonymy,
where the same tag may have different meanings. For exam-
ple, the “tiger” tag could be applied to the mammal or to
Apple computer’s operating system. Searching on the tag
“tiger” will return many images unrelated the carnivorous
mammals, requiring the user to sift through possibly a large
amount of irrelevant content. Another problem related to
homonymy is that of polysemy, which arises when a word
has multiple related meanings, such as “apple” to mean the
company or any of its products. Another problem is that
1http://del.icio.us
of synonymy, or multiple words having the same or related
meaning, for example, “baby” and “infant.” The problem
here is that if the user wants all images of young children
in their first year of life, searching on the tag “baby” may
not return all relevant images, since other users may have
tagged similar photographs with “infant.” Of course, plurals
(“tigers” vs ”tiger”) and many other tagging idiosyncrasies
(”myson” vs “son”) may also confound a tagging system.
Golder and Huberman identify yet another problem that
arises when using tags for categorization — that of the “ba-
sic level.” A given item can be described by terms along
a spectrum of specificity, ranging from specific to general.
A Siberian tiger can be described as a “tiger,” but also as
a “mammal” and “animal.” The basic level is the category
people choose for an object when communicating to others
about it. Thus, for most people, the basic level for canines
is “dog,” not the more general “animal” or the more specific
“beagle.” However, what constitutes the basic level varies
between individuals, and to a large extent depends on the
degree of expertise. To a dog expert, the basic level may be
the more specific “beagle” or “poodle,” rather than “dog.”
The basic level problem arises when different users choose
to describe the item at different levels of specificity. For ex-
ample, a dog expert tags an image of a beagle as “beagle,”
whereas the average user may tag a similar image as “dog.”
Unless the user is aware of the basic level variation and sup-
plies more specific (and more general) keywords during tag
search, he may miss a large number of relevant images.
Despite these problems, tagging is a light weight, flexi-
ble categorization system. The growing number of tagged
images provides evidence that users are adopting tag-
ging on Flickr (Marlow et al. 2006). There is specula-
tion (Mika 2005) that collective tagging will lead to a com-
mon informal classification system, dubbed a “folksonomy,”
that will be used to organize all information from all users.
Developing value-added systems on top of tags, e.g., which
allow users to better browse or search for relevant items, will
only accelerate wider acceptance of tagging.
Anatomy of Flickr
Flickr consists of a collection of interlinked user, photo, tag
and group pages. A typical Flickr photo page is shown in
Figure 1. It provides a variety of information about the im-
age: who uploaded it and when, what groups it has been sub-
mitted to, its tags, who commented on the image and when,
how many times the image was viewed or bookmarked as a
“favorite.” Clicking on a user’s name brings up that user’s
photo stream, which shows the latest photos she has up-
loaded, the images she marked as “favorite,” and her profile,
which gives information about the user, including a list of
her contacts and groups she belong to. Clicking on the tag
shows user’s images that have been tagged with this key-
word, or all public images that have been similarly tagged.
Finally, the group link brings up the group’s page, which
shows the photo group, group membership, popular tags,
discussions and other information about the group.
Figure 1: A typical photo page on Flickr
Groups Flickr allows users to create special interest
groups on any imaginable topic. There are groups for
showcasing exceptional images, group for images of circles
within a square, groups for closeups of flowers, for the color
red (and every other color and shade), groups for rating sub-
mitted images, or those used solely to generate comments.
Some groups are even set up as games, such as The Infinite
Flickr, where the rule is that a user post an image of her-
self looking at the screen showing the last image (of a user
looking at the screen showing next to last image, etc).
There is redundancy and duplication in groups. For exam-
ple, groups for child photography include Children’s Por-
traits, Kidpix, Flickr’s Cutest Kids, Kids in Action, Tod-
dlers, etc. A user chooses one, or usually several, groups to
which to submit an image. We believe that group names can
be viewed as a kind of publicly agreed upon tags.
Contacts Flickr allows users to designate others as friends
or contacts and makes it easy to track their activities. A
single click on the “Contacts” hyperlink shows the user the
latest images from his or her contacts. Tracking activities of
friends is a common feature of many social media sites and
is one of their major draws.
Interestingness Flickr uses the “interestingness” criterion
to evaluate the quality of the image. Although the algorithm
that is used to compute this is kept secret to prevent gaming
the system, certain metrics are taken into account: “where
the clickthroughs are coming from; who comments on it and
when; who marks it as a favorite; its tags and many more
things which are constantly changing.”2
Browsing and searching
Flickr offers the user a number of browsing and searching
methods. One can browse by popular tags, through the
groups directory, through the Explore page and the calen-
dar interface, which provides access to the 500 most “inter-
esting” images on any given day. A user can also browse
geotagged images through the recently introduced map in-
terface. Finally, Flickr allows for social browsing through
the “Contacts” interface that shows in one place the recent
images uploaded by the user’s designated contacts.
Flickr allows searching for photos using full text or tag
search. A user can restrict the search to all public photos,
his or her own photos, photos she marked as her favorite, or
photos from a specific contact. The advanced search inter-
face currently allows further filtering by content type, date
and camera.
Search results are by default displayed in reverse chrono-
logical order of being uploaded, with the most recent images
on top. Another available option is to display images by their
“interestingness” value, with the most “interesting” images
on top.
2http://flickr.com/explore/interesting/
Personalizing search results
Suppose a user is interested in wildlife photography and
wants to see images of tigers on Flickr. The user can search
for all public images tagged with the keyword “tiger.” As
of March 2007, such a search returns over 55, 500 results.
When images are arranged by their “interestingness,” the
first page of results contains many images of tigers, but
also of a tiger shark, cats, butterfly and a fish. Subsequent
pages of search results show, in addition to tigers, children in
striped suits, flowers (tiger lily), more cats, Mac OS X (tiger)
screenshots, golfing pictures (Tiger Woods), etc. In other
words, results include many false positives, images that are
irrelevant to what the user had in mind when executing the
search.
We assume that when the search term is ambiguous, the
sense that the user has in mind is related to her interests. For
example, when a child photographer is searching for pictures
of a “newborn,” she is most likely interested in photographs
of human babies, not kittens, puppies, or ducklings. Simi-
larly, a nature photographer specializing in macro photogra-
phy is likely to be interested in insects when searching on
the keyword “beetle,” not a Volkswagen car. Users express
their photography preferences and interests in a number of
ways on Flickr. They express them through their contacts
(photographers they choose to watch), through the images
they upload to Flickr, through the tags they add to these im-
ages, through the groups they join, and through the images
of other photographers they mark as their favorite. In this pa-
per we show that we can personalize results of tag search by
exploiting information about user’s preferences. In the sec-
tions below, we describe two search personalization meth-
ods: one that relies on user-created tags and one that exploits
user’s contacts. We show that both methods improve search
performance by reducing the number of false positives, or
irrelevant results, returned to the user.
Data collections
To show how user-created metadata can be used to personal-
ize results of tag search, we retrieved a variety of data from
Flickr using their public API.
Data sets
We collected images by performing a single keyword tag
search of all public images on Flickr. We specified that the
returned images are ordered by their “interestingness” value,
with most interesting images first. We retrieved the links to
the top 4500 images for each of the following search terms:
tiger possible senses include (a) big cat ( e.g., Asian tiger),
(b) shark (Tiger shark), (c) flower (Tiger Lily), (d) golfing
(Tiger Woods), etc.
newborn possible senses include (a) a human baby, (b) kit-
ten, (c) puppy, (d) duckling, (e) foal, etc.
beetle possible senses include (a) a type of insect and (b)
Volkswagen car model
For each image in the set, we used Flickr’s API to retrieve
the name of the user who posted the image (image owner),
and all the image’s tags and groups.
query relevant not relevant precision
newborn 412 83 0.82
tiger 337 156 0.67
beetle 232 268 0.46
Table 1: Relevance results for the top 500 images retrieved
by tag search
Users
Our objective is to personalize tag search results; therefore,
to evaluate our approach, we need to have users to whose
interests the search results are being tailored. We identi-
fied four users who are interested in the first sense of each
search term. For the newborn data set, those users were one
of the authors of the paper and three other contacts within
that user’s social network who were known to be interested
in child photography. For the other datasets, the users were
chosen from among the photographers whose images were
returned by the tag search. We studied each user’s profile
to confirm that the user was interested in that sense of the
search term. We specifically looked at group membership
and user’s tags. Thus, for the tiger data set, groups that
pointed to the user’s interest in P. tigris were Big Cats, Zoo,
The Wildlife Photography, etc. In addition to group mem-
bership, tags that pointed to user’s interest in a topic, e.g., for
the beetle data set, we assumed that users who used tags na-
ture and macro were interested in insects rather than cars.
Likewise, for the newborn data set, users who had uploaded
images they tagged with baby and child were probably in-
terested in human newborns.
For each of the twelve users, we collected the names of
their contacts, or Level 1 contacts. For each of these con-
tacts, we also retrieved the list of their contacts. These are
called Level 2 contacts. In addition to contacts, we also re-
trieved the list of all the tags, and their frequencies, that the
users had used to annotate their images. In addition to all
tags, we also extracted a list of related tags for each user.
These are the tags that appear together with the tag used as
the search term in the user’s photos. In other words, suppose
a user, who is a child photographer, had used tags such as
“baby”, “child”, “newborn”, and “portrait” in her own im-
ages. Tags related to newborn are all the tags that co-occur
with the “newborn” tag in the user’s own images. This in-
formation was also extracted via Flickr’s API.
Search results
We manually evaluated the top 500 images in each data set
and marked each as relevant if it was related to the first sense
of the search term listed above, not relevant or undecided, if
the evaluator could not understand the image well enough to
judge its relevance.
In Table 1, we report the precision of the search within the
500 labeled images, as judged from the point of view of the
searching users. Precision is defined as the ratio of relevant
images within the result set over the 500 retrieved images.
Precision of tag search on these sample queries is not very
high due to the presence of false positives — images not rel-
evant to the sense of the search term the user had in mind.
In the sections below we show how to improve search per-
formance by taking into consideration supplementary infor-
mation about user’s interests provided by her contacts and
tags.
Personalizing by contacts
Flickr encourages users to designate others as contacts by
making is easy to view the latest images submitted by them
through the “Contacts” interface. Users add contacts for a
variety of reasons, including keeping in touch with friends
and family, as well as to track photographers whose work
is of interest to them. We claim that the latter reason is the
most dominant of the reasons. Therefore, we view user’s
contacts as an expression of the user’s interests. In this sec-
tion we show that we can improve tag search results by filter-
ing through the user’s contacts. To personalize search results
for a particular user, we simply restrict the images returned
by the tag search to those created by the user’s contacts.
Table 2 shows how many of the 500 images in each data
set came from a user’s contacts. The column labeled “# L1”
gives the number of user’s Level 1 contacts. The follow-
ing columns show how many of the images were marked as
relevant or not relevant by the filtering method, as well as
precision and recall relative to the 500 images in each data
set. Recall measures the fraction of relevant retrieved im-
ages relative to all relevant images within the data set. The
last column “improv” shows percent improvement in preci-
sion over the plain (unfiltered) tag search.
As Table 2 shows, filtering by contacts improves the pre-
cision of tag search for most users anywhere from 22% to
over 100% when compared to plain search results in Ta-
ble 1. The best performance is attained for users within the
newborn set, with a large number of relevant images cor-
rectly identified as being relevant, and no irrelevant images
admitted into the result set. The tiger set shows an average
precision gain of 42% over four users, while the beetle set
shows an 85% gain.
Increase in precision is achieved by reducing the number
of false positives, or irrelevant images that are marked as rel-
evant by the search method. Unfortunately, this gain comes
at the expense of recall: many relevant images are missed
by this filtering method. In order to increase recall, we en-
large the contacts set by considering two levels of contacts:
user’s contacts (Level 1) and her contacts’ contacts (Level
2). The motivation for this is that if the contact relation-
ship expresses common interests among users, user’s inter-
ests will also be similar to those of her contacts’ contacts.
The second half of Table 2 shows the performance of
filtering the search results by the combined set of user’s
Level 1 and Level 2 contacts. This method identifies many
more relevant images, although it also admits more irrele-
vant images, thereby decreasing precision. This method still
shows precision improvement over plain search, with pre-
cision gain of 9%, 16% and 11% respectively for the three
data sets.
Personalizing by tags
In addition to creating lists of contacts, users express their
photography interests through the images they post on
Flickr. We cannot yet automatically understand the content
of images. Instead, we turn to the metadata added by the
user to the image to provide a description of the image. The
metadata comes in a variety of forms: image title, descrip-
tion, comments left by other users, tags the image owner
added to it, as well as the groups to which she submitted the
image. As we described in the paper, tags are useful im-
age descriptors, since they are used to categorize the image.
Similarly, group names can be viewed as public tags that a
community of users have agreed on. Submitting an image to
a group is, therefore, equivalent to tagging it with a public
In the section below we describe a probabilistic model
that takes advantage of the images’ tag and group informa-
tion to discover latent topics in each search set. The users’
interests can similarly be described by collections of tags
they had used to annotate their own images. The latent top-
ics found by the model can be used to personalize search
results by finding images on topics that are of interest to a
particular user.
Model definition
We need to consider four types of entities in the model: a
set of users U = {u1, ..., un}, a set of images or photos
I = {i1, ..., im}, a set of tags T = {t1, ..., to}, and a set
of groups G = {g1, ..., gp}. A photo ix posted by owner
ux is described by a set of tags {tx1, tx2, ...} and submitted
to several groups {gx1, gx2, ...}. The post could be viewed
as a tuple < ix, ux, {tx1, tx2, ...}, {gx1, gx2, ...} >. We as-
sume that there are n users, m posted photos and p groups
in Flickr. Meanwhile, the vocabulary size of tags is q. In
order to filter images retrieved by Flickr in response to tag
search and personalize them for a user u, we compute the
conditional probability p(i|u), that describes the probability
that the photo i is relevant to u based on her interests. Im-
ages with high enough p(i|u) are then presented to the user
as relevant images.
As mentioned earlier, users choose tags from an uncon-
trolled vocabulary according to their styles and interests.
Images of the same subject could be tagged with different
keywords although they have similar meaning. Meanwhile,
the same keyword could be used to tag images of different
subjects. In addition, a particular tag frequently used by one
user may have a different meaning to another user. Proba-
bilistic models offer a mechanism for addressing the issues
of synonymy, polysemy and tag sparseness that arise in tag-
ging systems.
We use a probabilistic topic
model (Rosen-Zvi et al. 2004) to model user’s image
posting behavior. As in a typical probabilistic topic model,
topics are hidden variables, representing knowledge cate-
gories. In our case, topics are equivalent to image owner’s
interests. The process of photo posting by a particular user
could be described as a stochastic process:
• User u decides to post a photo i.
user # L1 rel. not rel. Pr Re improv # L2+L2 rel. not rel. Pr Re improv
newborn
user1 719 232 0 1.00 0.56 22% 49,539 349 62 0.85 0.85 4%
user2 154 169 0 1.00 0.41 22% 10,970 317 37 0.9 0.77 10%
user3 174 147 0 1.00 0.36 22% 13,153 327 39 0.89 0.79 9%
user4 128 132 0 1.00 0.32 22% 8,439 310 29 0.91 0.75 11%
tiger
user5 63 11 1 0.92 0.03 37% 13,142 255 71 0.78 0.76 16%
user6 103 78 3 0.96 0.23 44% 14,425 266 83 0.76 0.79 13%
user7 62 65 1 0.98 0.19 47% 7,270 226 60 0.79 0.67 18%
user8 56 30 0 0.97 0.09 44% 7,073 240 63 0.79 0.71 18%
beetle
user9 445 18 1 0.95 0.08 106% 53,480 215 221 0.49 0.93 7%
user10 364 35 8 0.81 0.15 77% 41,568 208 217 0.49 0.90 7%
user11 783 78 25 0.75 0.34 65% 62,610 218 227 0.49 0.94 7%
user12 102 7 1 0.88 0.03 90% 14,324 163 152 0.52 0.70 13%
Table 2: Results of filtering tag search by user’s contacts. “# L1” denotes the number of Level 1 contacts and “# L1+L2” shows
the number of Level 1 and Level 2 contacts, with the succeeding columns displaying filtering results of that method: the number
of images marked relevant or not relevant, as well as precision and recall of the filtering method relative to the top 500 images.
The columns marked “improv” show improvement in precision over plain tag search results.
Figure 2: Graphical representation for model-based infor-
mation filtering. U , T , G and Z denote variables “User”,
“Tag”, “Group”, and “Topic” respectively. Nt represents a
number of tag occurrences for a one photo (by the photo
owner); D represents a number of all photos on Flickr.
Meanwhile, Ng denotes a number of groups for a particu-
lar photo.
• Based on user u’s interests and the subject of the photo, a
set of topics z are chosen.
• Tag t is then selected based on the set of topics chosen in
the previous state.
• In case that u decides to expose her photo to some groups,
a group g is then selected according to the chosen topics.
The process is depicted in a graphical form in Figure 2.
We do not treat the image i as a variable in the model but
view it as a co-occurrence of a user, a set of tags and a set of
groups. From the process described above, we can represent
the joint probability of user, tag and group for a particular
photo as
p(i) = p(ui, Ti, Gi)
= p(ui) ·
p(zk|ui)p(ti|z)
)ni(t)
p(zk|ui)p(gi|z)
)ni(g)
Note that it is straightforward to exclude photo’s group
information from the above equation simply by omitting the
terms relevant to g. nt and ng is a number of all possible tags
and groups respectively in the data set. Meanwhile, ni(t)
and ni(g) act as indicator functions: ni(t) = 1 if an image i
is tagged with tag t; otherwise, it is 0. Similarly, ni(g) = 1
if an image i is submitted to group g; otherwise, it is 0. k is
the predefined number of topics.
The joint probability of photos in the data set I is defined
p(I) =
p(im).
In order to estimate parameters p(z|ui), p(ti|z), and p(gi|z),
we define a log likelihood L, which measures how the esti-
mated parameters fit the observed data. According to the
EM algorithm (Dempster et al. 1977), L will be used as an
objective function to estimate all parameters. L is defined as
L(I) = log(p(I)).
In the expectation step (E-step), the joint probability of
the hidden variable Z given all observations is computed
from the following equations:
p(z|t, u) ∝ p(z|u) · p(t|z) (1)
p(z|g, u) ∝ p(z|u) · p(g|z). (2)
L cannot be maximized easily, since the summation over
the hidden variable Z appears inside the logarithm. We in-
stead maximize the expected complete data log-likelihood
over the hidden variable, E[Lc], which is defined as
E[Lc] =
log(p(u))
ni(t) ·
p(z|u, t) (log(p(z|u)· (t|z))
ni(g) ·
p(z|u, g) (log(p(z|g)· (g|z))
Since the term
log(p(ui)) is not related to parame-
ters and can be computed directly from the observed data,
we discard this term from the expected complete data log-
likelihood. With normalization constraints on all parame-
ters, Lagrange multipliers τ , ρ, ψ are added to the expected
log likelihood, yielding the following equation
H = E[Lc] +
p(t|z)
p(g|z)
p(z|u)
We maximize H with respect to p(t|zk), p(g|zk), and
p(zk|u), and then eliminate the Lagrange multipliers to ob-
tain the following equations for the maximization step:
p(t|z) ∝
ni(t) · p(z|t, u) (3)
p(g|z) ∝
ni(g) · p(z|g, u) (4)
p(zk|um) ∝
nm(t) · p(zk|um, t) (5)
nm(g) · p(zk|um, g)
The algorithm iterates between E and M step until the log
likelihood for all parameter values converge.
Model-based personalization
We can use the model developed in the previous section to
find the images i most relevant to the interests of a partic-
ular user u′. We do so by learning the parameters of the
model from the data and using these parameters to compute
the conditional probability p(i|u′). This probability can be
factorized as follows:
p(i|u′) =
p(ui, Ti, Gi|z) · p(z|u
′) , (6)
where ui is the owner of image i in the data set, and Ti and
Gi are, respectively, the set of all the tags and groups for the
image i.
The former term in Equation 6 can be factorized further
p(ui, Ti, Gi|z) ∝ p(Ti|z)· (Gi|z)· (z|ui) · p(ui)
p(ti|z)
p(gi|z)
· p(z|ui) · p(ui) .
We can use the learned parameters to compute this term di-
rectly.
We represent the interests of user u′ as an aggregate of the
tags that u′ had used in the past for tagging her own images.
This information is used to to approximate p(z|u′):
p(z|u′) ∝
n(t′ = t) · p(z|t)
where n(t′ = t) is a frequency (or weight) of tag t′ used
by u′. Here we view n(t′ = t) is proportional to p(t′|u′).
Note that we can use either all the tags u′ had applied to the
images in her photostream, or a subset of these tags, e.g.,
only those that co-occur with some tag in user’s images.
Evaluation
We trained the model separately on each data set of 4500
images. We fixed the number of topics at ten. We then eval-
uated our model-based personalization framework by using
the learned parameters and the information about the in-
terests of the selected users to compute p(i|u′) for the top
500 (manually labeled) images in the set. Information about
user’s interests was captured either by (1) all tags (and their
frequencies) that are used in all the images of the user’s pho-
tostream or (2) related tags that occurred in images that were
tagged with the search keyword (e.g., “newborn”) by the
user.
Computation of p(t|z) is central to the parameter estima-
tion process, and it tells us something about how strongly a
tag t contributes to a topic z. Table 3 shows the most prob-
able 25 tags for each topic for the tiger data set trained on
ten topics. Although the tag “tiger” dominates most topics,
we can discern different themes from the other tags that ap-
pear in each topic. Thus, topic z5 is obviously about domes-
tic cats, while topic z8 is about Apple computer products.
Meanwhile, topic z2 is about flowers and colors (“flower,”
“lily,” “yellow,” “pink,” “red”); topic z6 is about about
places (“losangeles,” “sandiego,” “lasvegas,” “stuttgard,”),
presumably because these places have zoos. Topic z7 con-
tains several variations of tiger’s scientific name, “panthera
tigris.” This method appears to identify related words well.
Topic z5, for example, gives synonyms “cat,” “kitty,” as well
as the more general term “pet” and the more specific terms
“kitten” and “tabby.” It even contains the Spanish version of
the word: “gatto.” In future work we plan to explore using
this method to categorize photos in a more abstract way. We
also note that related terms can be used to increase search
recall by providing additional keywords for queries.
Table 4 presents results of model-based personalization
for the case that uses information from all of user’s tags.
The model was trained with ten topics. Results are pre-
sented for different thresholds. The first two columns, for
example, report precision and recall for a high threshold that
z1 z2 z3 z4 z5
tiger tiger tiger tiger tiger
zoo specanimal cat thailand cat
animal animalkingdomelite kitty bengal animal
nature abigfave cute animals animals
animals flower kitten tigers zoo
wild butterfly cats canon bigcat
tijger macro orange d50 cats
wildlife yellow eyes tigertemple tigre
ilovenature swallowtail pet 20d animalplanet
cub lily tabby white tigers
siberiantiger green stripes nikon bigcats
blijdorp canon whiskers kanchanaburi whitetiger
london insect white detroit mammal
australia nature art life wildlife
portfolio pink feline michigan colorado
white red fur detroitzoo stripes
dierentuin flowers animal eos denver
toronto orange gatto temple sumatrantiger
stripes eastern pets park white
amurtiger usa black asia feline
nikonstunninggallery impressedbeauty paws ball mammals
s5600 tag2 furry marineworld sumatran
eyes specnature nose baseball exoticcats
sydney black teeth detroittigers exoticcat
cat streetart beautiful wild big
z6 z7 z8 z9 z10
tiger nationalzoo tiger tiger tiger
tigers tiger apple india lion
dczoo sumatrantiger mac canon dog
tigercub zoo osx wildlife shark
california nikon macintosh impressedbeauty nyc
lion washingtondc screenshot endangered cat
cat smithsonian macosx safari man
cc100 washington desktop wildanimals people
florida animals imac wild arizona
girl cat stevejobs tag1 rock
wilhelma bigcat dashboard tag3 beach
self tigris macbook park sand
lasvegas panthera powerbook taggedout sleeping
stuttgart bigcats os katze tree
me d70s 104 nature forest
baby pantheratigrissumatrae canon bravo puppy
tattoo dc x nikon bird
endangered sumatrae ipod asia portrait
illustration animal computer canonrebelxt marwell
?? 2005 ibook bandhavgarh boy
losangeles pantheratigris intel vienna fish
portrait nikond70 keyboard schnbrunn panther
sandiego d70 widget zebra teeth
lazoo 2006 wallpaper pantheratigris brooklyn
giraffe topv111 laptop d2x bahamas
Table 3: Top tags ordered by p(t—z) for the ten topic model of the “tiger” data set.
Pr Re Pr Re Pr Re Pr Re Pr Re
newborn
n=50 n=100 n=200 n=300 n=412*
user1 1.00 0.12 1.00 0.24 1.00 0.49 0.94 0.68 0.89 0.89
user2 1.00 0.12 1.00 0.24 1.00 0.49 0.92 0.67 0.87 0.87
user3 1.00 0.12 0.88 0.21 0.84 0.41 0.85 0.62 0.89 0.89
user4 1.00 0.12 0.99 0.24 1.00 0.48 0.94 0.69 0.89 0.89
tiger
n=50 n=100 n=200 n=300 n=337*
user5 0.94 0.14 0.90 0.27 0.82 0.48 0.80 0.71 0.79 0.79
user6 0.76 0.11 0.80 0.24 0.79 0.47 0.77 0.69 0.77 0.77
user7 0.94 0.14 0.90 0.27 0.82 0.48 0.80 0.71 0.79 0.79
user8 0.90 0.13 0.88 0.26 0.82 0.49 0.79 0.71 0.79 0.79
beetle
n=50 n=100 n=200 n=232* n=300
user9 1.00 0.22 0.99 0.43 0.77 0.66 0.70 0.70 0.66 0.85
user10 0.98 0.21 0.99 0.43 0.77 0.66 0.70 0.70 0.66 0.85
user11 0.98 0.21 0.93 0.40 0.50 0.43 0.51 0.51 0.50 0.65
user12 1.00 0.22 0.99 0.43 0.77 0.66 0.70 0.70 0.66 0.85
Table 4: Filtering results where a number of learned topics is 10, excluding group information, and user’s personal information
obtained from all tags she used for her photos. Asterisk denotes R-precision of the method, or precision of the first n results,
where n is the number of relevant results in the data set.
marks only the 50 most probable images as relevant. The re-
maining 450 images are marked as not relevant to the user.
Recall is low, because many relevant images are excluded
from the results for such a high threshold. As the thresh-
old is decreased (n = 100, n = 200, . . .), recall relative to
the 500 labeled images increases. Precision remains high in
all cases, and higher than precision of the plain tag search
reported in Table 1. In fact, most of the images in the top
100 results presented to the user are relevant to her query.
The column marked with the asterisk gives the R-precision
of the method, or precision of the first R results, where R is
the number of relevant results. The average R-precision of
this filtering method is 8%, 17% and 42% better than plain
search precision on our three data sets.
Performance results of the approach that uses related tags
instead of all tags are given in Table 5. We explored this di-
rection, because we believed it could help discriminate be-
tween different topics that interest a user. Suppose, a child
photographer is interested in nature photography as well as
child portraiture. The subset of tags he used for tagging his
“newborn” portraits will be different from the tags used for
tagging nature images. These tags could be used to differen-
tiate between newborn baby and newborn colt images. How-
ever, on the set of users selected for our study, using related
tags did not appear to improve results. This could be be-
cause the tags a particular user used together with, for ex-
ample, “beetle” do not overlap significantly with the rest of
the data set.
Including group information did not significantly improve
results (not presented in this manuscript). In fact, group in-
formation sometimes hurts the estimation rather than helps.
We believe that this is because our data sets (sorted by Flickr
according to image interestingness) are biased by the pres-
ence of general topic groups (e.g., Search the Best, Spec-
tacular Nature, Let’s Play Tag, etc.). We postulate that
group information would help estimate p(i|z) in cases where
the photo has few or no tags. Group information would help
filling in the missing data by using group name as another
tag. We also trained the model on the data with 15 topics,
but found no significant difference in results.
Previous research
Recommendation or personalization systems can be cate-
gorized into two main categories. One is collaborative fil-
tering (Breese et al. 1998) which exploits item ratings from
many users to recommend items to other like-minded users.
The other is content-based recommendation, which relies
on the contents of an item and user’s query, or other user
information, for prediction (Mooney and Roy 2000). Our
first approach, filtering by contacts, can be viewed as im-
plicit collaborative filtering, where the user–contact rela-
tionship is viewed as a preference indicator: it assumes
that the user likes all photos produced by her contacts. In
our previous work, we showed that users do indeed agree
with the recommendations made by contacts (Lerman 2007;
Lerman and Jones 2007). This is similar to the ideas imple-
mented by MovieTrust (Golbeck 2006), but unlike that sys-
tem, social media sites do not require users to rate their trust
in the contact.
Meanwhile, our second approach, filtering by tags (and
groups), shares some characteristics with both methods. It
is similar to collaborative filtering, since we use tags to rep-
resent agreement between users. It is also similar to content-
based recommendation, because we represent image content
by the tags and group names that have been assigned to it by
the user.
Our model-based filtering system is technically similar to,
but conceptually different from, probabilistic models pro-
Pr Re Pr Re Pr Re Pr Re Pr Re
newborn
n=50 n=100 n=200 n=300 n=412*
Pr Re Pr Re Pr Re Pr Re Pr Re
user1 0.8 0.10 0.78 0.19 0.79 0.38 0.77 0.56 0.79 0.79
user2 0.8 0.10 0.82 0.20 0.80 0.39 0.77 0.56 0.83 0.83
user3 0.98 0.12 0.88 0.21 0.84 0.41 0.80 0.58 0.85 0.85
user4 0.98 0.12 0.88 0.21 0.84 0.41 0.85 0.62 0.88 0.88
tiger
n=50 n=100 n=200 n=300 n=337*
user5 0.84 0.12 0.86 0.26 0.78 0.46 0.78 0.69 0.77 0.77
user6 0.72 0.11 0.79 0.23 0.78 0.46 0.76 0.68 0.76 0.76
user7 0.72 0.11 0.78 0.23 0.78 0.46 0.76 0.68 0.76 0.76
user8 0.9 0.13 0.82 0.24 0.80 0.47 0.78 0.69 0.78 0.78
beetle
n=50 n=100 n=200 n=232* n=300
user9 0.78 0.17 0.62 0.27 0.58 0.50 0.54 0.54 0.53 0.68
user10 0.98 0.21 0.88 0.38 0.77 0.66 0.72 0.72 0.65 0.84
user11 0.96 0.21 0.74 0.32 0.62 0.53 0.59 0.59 0.56 0.72
user12 0.98 0.21 0.99 0.43 0.77 0.66 0.70 0.70 0.66 0.85
Table 5: Filtering results where a number of learned topics is 10, excluding group information, and user’s personal information
obtained from all tags she used for her photos, which are tagged by the search term
posed by (Popescul et al. 2001). Both models are proba-
bilistic generative models that describe co-occurrences of
users and items of interest. In particular, the model assumes
a user generates her topics of interest; then the topics gen-
erate documents and words in those documents if the user
prefers those documents. In our model, we metaphorically
assume the photo owner generates her topics of interest. The
topics, in turn, generate tags that the owner used to annotate
her photo. However, unlike the previous work, we do not
treat photos as variables, as they do for documents. This is
because images are tagged only by their owners; meanwhile,
in their model, all users who are interested in a document
generate topics for that document.
Our model-based approach is almost identical to the
author-topic model(Rosen-Zvi et al. 2004). However, we
extend their framework to address (1) how to exploit photo’s
group information for personalized information filtering; (2)
how to approximate user’s topics of interest from partially
observed personal information (the tags the user used to de-
scribe her own images). For simplicity, we use the classi-
cal EM algorithm to train the model; meanwhile they use a
stochastic approximation approach due to the difficulty in-
volved in performing exact an inference for their generative
model.
Conclusions and future work
We presented two methods for personalizing results of im-
age search on Flickr. Both methods rely on the meta-
data users create through their everyday activities on Flickr,
namely user’s contacts and the tags they used for annotating
their images. We claim that this information captures user’s
tastes and preferences in photography and can be used to
personalize search results to the individual user. We showed
that both methods dramatically increase search precision.
We believe that increasing precision is an important goal for
personalization, because dealing with the information over-
load is the main issue facing users, and we can help users
by reducing the number of irrelevant results the user has to
examine (false positives). Having said that, our tag-based
approach can also be used to expand the search by suggest-
ing relevant related keywords (e.g., “pantheratigris,” “big-
cat” and ”cub” for the query tiger).
In addition to tags and contacts, there exists other meta-
data, favorites and comments, that can be used to aid infor-
mation personalization and discovery. In our future work
we plan to address the challenge of combing these heteroge-
neous sources of evidence within a single approach. We will
begin by combining contacts information with tags.
The probabilistic model needs to be explored further.
Right now, there is no principled way to pick the number
of latent topics that are contained in a data set. We also plan
to have a better mechanism for dealing with uninformative
tags and groups. We would like to automatically identify
general interest groups, such as the Let’s Play Tag group,
that do not help to discriminate between topics.
The approaches described here can be applied to other so-
cial media sites, such as Del.icio.us. We imagine that in
near future, all of Web will be rich with metadata, of the sort
described here, that will be used to personalize information
search and discovery to the individual user.
Acknowledgements
This research is based on work supported in part by the Na-
tional Science Foundation under Award Nos. IIS-0535182
and in part by DARPA under Contract No. NBCHD030010.
The U.S.Government is authorized to reproduce and dis-
tribute reports for Governmental purposes notwithstanding
any copyright annotation thereon. The views and conclu-
sions contained herein are those of the authors and should
not be interpreted as necessarily representing the official
policies or endorsements, either expressed or implied, of
any of the above organizations or any person connected with
them.
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	Introduction
	Tagging for organizing images
	Anatomy of Flickr
	Browsing and searching
	Personalizing search results
	Data collections
	Data sets
	Users
	Search results
	Personalizing by contacts
	Personalizing by tags
	Model definition
	Model-based personalization
	Evaluation
	Previous research
	Conclusions and future work
