Abstract:
A method for uniquely identifying an initial exposure of a visitor to an advertisement campaign is provided. The method includes causing a web browser to call an overlap server upon serving of an advertisement of the advertisement campaign; storing, by the overlap server, an overlap data structure in any one of the web browser and a web server, wherein the overlap data structure is coded to identify the advertisement campaign, a host website through which the visitor is exposed to the advertisement campaign, and an indication as to whether the advertisement campaign was previously visited by the visitor of the web browser; and processing, by the overlap server, the overlap data structure to determine the initial exposure of the visitor to the advertisement of the advertisement campaign and the host website through which the visitor was exposed to the advertisement campaign.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of U.S. patent application Ser. No. 13/412,219, filed Mar. 5, 2012, now U.S. Pat. No. 8,533,321. The Ser. No. 13/412,219 is a continuation of U.S. patent application Ser. No. 11/641,062 filed Dec. 19, 2006, now U.S. Pat. No. 8,156,218, the contents of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the need to compute the total number of different visitors to objects, such as events, shops, journals, and the like. In particular, it relates to a method for tracking a cumulative number of identifiable visitors to different websites. 
     BACKGROUND OF THE INVENTION 
     There are many products and services where actions are conditioned on an aggregate total of events. For example, special offers and bonuses may be given based on an accumulated amount spent. Likewise, the rank of an item in a given hierarchy may be based on its relative popularity as measured by a number of visitors or purchasers and the like. When a desired action pertains to only one specific customer, for example a frequent flyer eligible for free flight tickets, it is straightforward to define a database structure where cumulative ticket price is stored for each member of the frequent flyers&#39; club so as to allow bonuses to be awarded when a member&#39;s cumulative ticket price exceeds a predetermined threshold. 
     It is more difficult to aggregate scores when the scores themselves are aggregates to which more than one item or user contributes. For example, US2003115586 (Lejouan et al.) discloses a method for measuring and analyzing activity of multiple terminals on a data communications network. Data exchanged over the communications network by the terminals is stored in a database for subsequent analysis. Thus data comprising at least descriptive data of each packet transmitted and received by each terminal, is collected and transmitted to a central server where aggregate databases are generated to supply data representing the activity of a panel of equipment items connected to the communications network. This allows actions to be taken based on aggregate behavior of a target audience. 
     In a typical Internet scenario, an advertisement is displayed on a number of different websites so that users who enter some or all of these websites are repeatedly exposed to that advertisement. In order to determine a single website that serves as the most effective vehicle for disseminating the advertisement, it is only necessary to survey and count the number of users surfing each website. 
     WO0133831 (Peroff et al) discloses an interactive web-based survey instrument and method that allows the application of different selection criteria to be applied at a subscriber website as a survey participant. Selection criteria may be applied at a subscriber website that is visited, such as random or periodic selection of visiting users. Moreover, the user&#39;s activities at the subscriber site may be monitored, such as number of web pages viewed, time spent viewing and so forth. 
     US2002082901 (Doron et al.) discloses a system and method for discovering relationships among items and for recommending items based on the discovered relationships. The recommendations are based on user profiles that take into account actual preferences of users, without requiring users to complete questionnaires. 
     US2005021440 (Dresden) provides a virtual system that assists in the procurement of advertising on an Internet vendor site for the sale of products or services. The system links to a user&#39;s financial package to get data on the products or services and allows the user to set financial parameters based on the desired financial goals related to the product and advertising. Performance data regarding advertising is accessed and financial rules generated which are applied to generate a target price for advertising or one or more products. The system can acquire advertising automatically or assist in the submission of bids in an auction of advertising. 
     But frequently advertisers run campaigns where an advertisement is accessible from multiple websites and they want to know which combination of websites is the most effective vehicle for disseminating the advertisement. This information cannot be derived merely from the cumulative numbers of users surfing each website since frequently the same user will see the advertisement several times on different websites. So what really interests the advertiser is whether some of the websites are practically redundant such that not displaying the advertisement on one or more websites will not noticeably detract from the total number of unique surfers who are exposed to the advertisement. 
     To determine this it is not sufficient merely to count the number of exposures to each advertisement. It is necessary to identify each user surfing all websites that display the advertisement, to determine for each website of interest a list of unique surfers and to use this information to determine the most cost-effective combination of websites whereby maximum exposure is achieved for the least cost. Presently, collection of this data is implemented in the form of log lists with every log specifying “user x; visited site y”. Such log lists require extensive processing to draw meaningful conclusions, thus requiring significant computation and computer resources such as memory and CPU time. 
     It would therefore be desirable to provide a simplified mechanism that allows the number of visitors to each website to be tracked in such a manner as to allow fast determination of the most effective combination of websites to produce a more cost-effective exposure of the campaign. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention there is provided a computer-implemented method for tracking a cumulative number of identifiable visitors to different objects (S 1 , S 2  . . . S N ), each of which may be visited by multiple visitors, said method comprising: 
     maintaining a data structure adapted to contain records relating to each of said objects (S 1 , S 2  . . . S N ) and to each combination of two or more of said objects (S 1 S 2 , S 1 S 3 , . . . S 1 S N , S 2 S 3 , S 2 S 4 , . . . S 2 S N , S 1 S 2 S 3 , S 1 S 3 S 4 , . . . S 1 S N-1 S N  etc.), 
     for each visitor visiting an object (S M ) for the first time:
         (i) determining a current combination of objects previously visited by said visitor;   (ii) if no objects have been previously visited by said visitor:
           a) if the data structure contains a record relating to said object (S M ), incrementing said record relating to said object (S M ); and   b) if the data structure does not contain a record relating to said object (S M ), creating a record relating to said object (S M ) and setting said record to 1;   
           (iii) if objects have been visited by said visitor, decrementing the record relating to said current combination and:
           a) if the data structure contains a record relating to the combination of said object (S M ) with the current combination, incrementing said record relating to the combination of said object (S M ) with the current combination; and   b) if the data structure does not contain a record relating to the combination of said object (S M ) with the current combination, creating a record relating to the combination of said object (S M ) with the current combination and setting said record to 1.   
               

     According to another aspect of the invention, order of visitation is important and the data structure is adapted to store records relating to each of said objects (S 1 , S 2  . . . S N ) and to each permutation of two or more of said objects (S 1 S 2 , S 2 S 1 , S 1 S 3 , S 3 S 1 , . . . S 1 S N , S N S 1 , S 2 S 3 , S 3 S 2 , S 2 S 4 , S 4 S 2 , . . . S 2 S N , S N S 2 , S 1 S 2 S 3 , S 1 S 3 S 2 , . . . S 1 S 3 S 4 , . . . S 1 S N-1 S N  etc.), the method including: 
     for each visitor visiting an object (S M ) for the first time:
         (i) determining a current permutation of objects previously visited by said visitor that conforms to a specified visitation order;   (ii) if no objects have been previously visited by said visitor:
           a) if the data structure contains a record relating to said object (S M ), incrementing said record relating to said object (S M );   b) if the data structure does not contain a record relating to said object (S M ), creating a record relating to said object (S M ) and setting said record to 1;   
           (iii) if objects have been previously visited by said visitor in the specified visitation order, decrementing the record relating to said current permutation and:
           a) if the data structure contains a record relating to the permutation of said object (S M ) with the current permutation in the specified visitation order, incrementing said record; and   b) if the data structure does not contain a record relating to the permutation of said object (S M ) with the current permutation in the specified visitation order, creating a record relating to the permutation of said object (S M ) with the current permutation in the specified visitation order and setting said record to 1.   
               

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention and to see how it may be carried out in practice, some embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a pictorial representation of a system according to the invention; 
         FIGS. 2 and 3  are flow diagrams showing the principal operations carried out by an overlap server in the system shown in  FIG. 1  in accordance with different embodiments of the invention; and 
         FIG. 4  is a block diagram showing functionality of the overlap server in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows pictorially a system  10  wherein a plurality of web users  11   a ,  11   b  and  11   c  surf the Internet  12  so as to access websites  13   a  to  13   e . An overlap server  14  to be described in more detail below is coupled to the Internet  12  and tracks all connections to any one of the websites  13   a  to  13   e  so as to detect and identify which of the web users  11   a ,  11   b  and  11   c  access each of the websites. To the extent that reference is made to the web users without regard to a specific one thereof, such web users will be referenced as  11 . In a typical scenario an advertising agent sets up an ad campaign for a customer whereby a campaign is displayed on multiple host websites, such as yahoo.com, msn.com, aol.com and so on, so that when a web user  11  enters any of these host websites he or she is exposed to the campaign. It should be noted that while the invention is described with regard to an ad campaign, what is actually displayed is an advertisement and the host web page ultimately provides a link to that advertisement. Thus, reference to an ad campaign is not intended to be limiting in any way. The customer pays the owners of the host websites for displaying the campaign in exactly the same manner that he would pay a newspaper or magazine publisher for displaying a campaign. The invention allows the customer to determine how many unique visitors are exposed to the campaign from each host website, thus allowing him to assess whether it is financially worthwhile to display the campaign in all of the host websites or whether sufficient coverage will be achieved by a reduced number of host websites, thus saving costs. 
     To this end the campaign advertisements are served by an ad server (not shown) that maintains a list of source URLs of each advertisement, whose (ad server&#39;s) link is embedded in the host web page in known manner by means of HTML tags. When a web user opens that web page, a call is made to the ad server, which further calls the appropriate URL of the advertisement to be displayed. 
     Within the call to the ad server, a call is made also to the overlap server  14 . When a web user enters the web page, thus displaying the ad on the web user&#39;s computer, the HTML tag calls the overlap server  14  thereby establishing brief connection between the web user&#39;s computer and the overlap server  14 . During the brief connection thus established, the overlap server  14  is able to store a cookie on the web user&#39;s computer that is unique to the overlap server  14  and identifies the web user, since it is stored on his or her computer. Likewise, if the cookie already exists, the overlap server  14  is able to edit it on the web user&#39;s computer. The cookie is formatted to allow the overlap server  14  to identify a specific campaign as well as each host website through which the user was exposed to the campaign. Thus using the cookie, the overlap server  14  is able to determine whether the web user to whom the cookie belongs (i.e. on whose computer the cookie is stored) has been previously exposed to the currently identified campaign. A suitable cookie structure is described below by way of non-limiting example. 
     Additionally, the overlap server  14  maintains a database  15  that is structured to allow direct determination of the cumulative number of different web users exposed to n different websites (S 1 , S 2  . . . S N ). For each ad campaign, the database  15  maintains a number of separate records for each of the n websites and for each unique combination of websites, i.e. each unique combination of two websites, each unique combination of three websites, and so on. It is easy to show that the number of unique combinations of k websites out of a total of n websites is: 
     
       
         
           
             
               n 
               ! 
             
             
               
                 
                   ( 
                   
                     n 
                     - 
                     k 
                   
                   ) 
                 
                 ! 
               
               ⁢ 
               
                 k 
                 ! 
               
             
           
         
       
     
     Consider, by way of example, an advertising campaign where an ad is displayed on four different websites, A, B, C and D. A user can visit any one of these websites only, or he may visit any two the websites, or any three the websites, or he may visit all four websites. It is easy to tabulate the combinations of possible websites as shown in Table I below: 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Series 
                 Combinations 
                 Formula 
                 Σ 
               
               
                   
                   
               
             
             
               
                   
                 Single 
                 A, B, C, D 
                 
                   
                     
                       
                         
                           4 
                           ! 
                         
                         
                           
                             
                               ( 
                               
                                 4 
                                 - 
                                 1 
                               
                               ) 
                             
                             ! 
                           
                           ⁢ 
                           
                             1 
                             ! 
                           
                         
                       
                     
                   
                 
                 4 
               
               
                   
                   
               
               
                   
                 Double 
                 AB, AC, AD, BC, BD, CD 
                 
                   
                     
                       
                         
                           4 
                           ! 
                         
                         
                           
                             
                               ( 
                               
                                 4 
                                 - 
                                 2 
                               
                               ) 
                             
                             ! 
                           
                           ⁢ 
                           
                             2 
                             ! 
                           
                         
                       
                     
                   
                 
                 6 
               
               
                   
                   
               
               
                   
                 Triple 
                 ABC, ABD, ACD, BCD 
                 
                   
                     
                       
                         
                           4 
                           ! 
                         
                         
                           
                             
                               ( 
                               
                                 4 
                                 - 
                                 3 
                               
                               ) 
                             
                             ! 
                           
                           ⁢ 
                           
                             3 
                             ! 
                           
                         
                       
                     
                   
                 
                 4 
               
               
                   
                   
               
               
                   
                 Quad 
                 ABCD 
                   
                 1 
               
               
                   
                   
               
             
          
         
       
     
     Thus, in general the maximum number of records that must be maintained in the database for an ad campaign displayed on n different websites where the order of visiting the sites is not important is given by: 
     
       
         
           
             
               
                 ∑ 
                 
                   k 
                   = 
                   1 
                 
                 
                   n 
                   - 
                   1 
                 
               
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                   n 
                   ! 
                 
                 
                   
                     
                       ( 
                       
                         n 
                         - 
                         k 
                       
                       ) 
                     
                     ! 
                   
                   ⁢ 
                   
                     k 
                     ! 
                   
                 
               
             
             + 
             1 
           
         
       
     
     If the order of visiting the sites is important, then records must be maintained in the database for each permutation, rather than merely for each combination. It is easy to tabulate the permutations of possible websites as shown in Table II below: 
     
       
         
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 Series 
                 Permutations 
                 Formula 
                 Σ 
               
               
                   
               
             
             
               
                 Single 
                 A, B, C, D 
                 
                   
                     
                       
                         
                           4 
                           ! 
                         
                         
                           
                             ( 
                             
                               4 
                               - 
                               1 
                             
                             ) 
                           
                           ! 
                         
                       
                     
                   
                 
                  4 
               
               
                   
               
               
                 Double 
                 AB, BA, AC, CA, AD, DA, BC, CB, BD, DB, CD, DC 
                 
                   
                     
                       
                         
                           4 
                           ! 
                         
                         
                           
                             ( 
                             
                               4 
                               - 
                               2 
                             
                             ) 
                           
                           ! 
                         
                       
                     
                   
                 
                 12 
               
               
                   
               
               
                 Triple 
                 ABC, ACB, BAC, BCA, ABD, ADB, BAD, BDA, ACD, ADC, CAD, CDA 
                 
                   
                     
                       
                         
                           4 
                           ! 
                         
                         
                           
                             ( 
                             
                               4 
                               - 
                               3 
                             
                             ) 
                           
                           ! 
                         
                       
                     
                   
                 
                 12 
               
               
                   
               
               
                 Quad 
                 ABCD, ABDC, ACBD, 
                 4! 
                 24 
               
               
                   
                 ACDB . . . 
               
               
                   
               
             
          
         
       
     
     Thus, in general the maximum number of records that must be maintained in the database for an ad campaign displayed on n different websites where the order of visiting the sites is important is given by: 
     
       
         
           
             
               
                 ∑ 
                 
                   k 
                   = 
                   1 
                 
                 
                   n 
                   - 
                   1 
                 
               
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                   n 
                   ! 
                 
                 
                   
                     ( 
                     
                       n 
                       - 
                       k 
                     
                     ) 
                   
                   ! 
                 
               
             
             + 
             
               n 
               ! 
             
           
         
       
     
     In either case, it is clear that the number of records that must be maintained in the database  15  for any given advertising campaign is a determinate function of the number of websites participating in the campaign and can thus be configured when the campaign is set up. Clearly this data can be maintained by other means such as memory, flat files, etc. and reference to database is non-limiting. It should also be noted that some embodiments of the invention allow records to be added to the database  15  on-the-fly as required. 
       FIGS. 2 and 3  are flow charts that show the principal operations carried out by the overlap server  14  in accordance with two different embodiments of the invention. In the first embodiment depicted in  FIG. 2 , order of visitation is unimportant, while the second embodiment depicted in  FIG. 3  takes order of visitation into consideration. Since both methods share a common basis, the embodiment shown in  FIG. 2  will first be described in detail and later supplementary details will be provided of the slight differences required to implement the second embodiment depicted in  FIG. 3 . 
     When a web user  11  enters into a website displaying a campaign advertisement that calls the overlap server  14 , brief communication is established between the overlap server  14  and the web user&#39;s computer. During the brief communication thus established, the overlap server  14  checks whether a cookie belonging to the overlap server  14  is stored on the web user&#39;s computer. If so, the overlap server  14  reads the cookie in order to determine whether this particular web user has been previously exposed to the campaign displayed in the currently accessed web page. As will be explained below, this requires that each campaign has a unique ID that is encoded within the cookie. If the cookie does not contain the ID of the current campaign, this means that this is the first time that the web user has been exposed to the campaign and the database record corresponding to the website currently accessed by the web user is incremented by 1. The cookie is likewise updated to reflect the ID of the website via which the web user was exposed to the campaign. 
     On the other hand, if the cookie does contain the ID of the current campaign, this means that the web user was already exposed to the campaign. In this case, he may have accessed the same website previously and/or he may have accessed a different website participating in the ad campaign. Which of these two scenarios is applicable is also encoded within the cookie. Thus, by way of simple example, a multi-bit binary website overlap code may be stored in the cookie, wherein the number of bits is equal to the number of websites participating in the ad campaign and wherein each bit is initially zero and it set to 1 when the corresponding website is visited by the web user. Thus, by parsing the binary representation of the site overlap code each bit provides an indication as to whether or not the corresponding website were visited by the web user. There are, of course, other ways than binary to encode the site data, and a different encoding scheme is presented in greater detail below. 
     Based on the site overlap code, the database is now updated as follows. First, the site overlap code shows which websites have been previously visited by the web user. Specifically, before the site overlap code is updated by setting the corresponding bit to 1, the site overlap code shows the current combination of websites in the present campaign that have been visited by the current web user. This allows direct translation to a unique database record. Thus referring again, to Table I, if websites A and D were previously visited, this translates to a unique database record entry whose value is set to the cumulative number of unique (i.e. unrepeated) visitors who have visited both A and D but no other websites in the campaign. If the current website is A or D then, of course, the corresponding bit in the site overlap code within the cookie will already be 1; and in this case no further action is required. On the other hand, if the current website is B or C then the corresponding bit in the site overlap code within the cookie will be 0; and in this case the following actions are required. First, the database record corresponding to the current site overlap code (before it is updated) i.e. “AD” is decremented by 1 and the database record corresponding to the new site overlap code (after updating) i.e. “ABD” or “ABC” (as appropriate) is incremented by 1. The site overlap code is also, of course, updated to show that the current user also visited site B or C. 
     It will thus be apparent that the site overlap code always provides an indication as to the current combination of websites visited by any given identifiable user, while the database  15  provides an indication as to the cumulative number of visits to each combination of websites participating in the ad campaign. Thus, in the simple example shown in Table 1, the database shows the cumulative number of visits to A, B, C, D on their own; to the double combinations AB, AC, AD, BC, BD, CD; to the triple combinations ABC, ABD, ACD, BCD and to the quad combination ABCD. Suppose that the record corresponding to AB is the same or very close in value to the record corresponding to ABCD. This would imply that the added cost benefit of displaying the campaign on websites C and D is probably not warranted since the same or significantly the same exposure is provided by displaying the campaign on websites A and B alone—and, of course, for much less cost. Likewise, if one of websites, say A, on its own is dominant so that all combinations containing A show significantly more visits than combinations not containing A, this may imply a cost-effective ad campaign can be realized by advertising on website A alone. 
     The above description assumes that the order of visiting websites is not important so that it does not matter whether the web user arrived at website B after first surfing A and then D or whether he first surfed D and then A and only then arrived at B. And, of course, once the cookie is updated, all that is known is that the web user surfed A, D and B there being no way to determine in which order he visited these websites. However, there may be occasions where the order in which the web user surfs the websites is important. For example, suppose in the above embodiment, it emerges that the dominant combination is AD suggesting that the ad campaign is effective even if the campaign is displayed on websites A and D only. This would require that the customer pays two websites A and D for the right to display the campaign on their websites. 
     But suppose now that the order in which the websites are visited is also recorded. This gives valuable additional information since if web users predominantly visit A before visiting D, this would imply that an effective ad campaign can be realized by displaying the campaign on A only. Thus, in such an application order may be significant but it will be understood that this may not necessarily be true in other applications. For example, suppose an on-line museum wishes to assess which artifacts or exhibits are most popular with the object of exhibiting more artifacts in those exhibits that are popular at the expense of those that are rarely visited. In this case, order may not be significant and the desired information can be determined by configuring the database so as to store records for each combination of artifacts visited—or, more likely, for each combination of exhibits. So, for example, if the curator of an on-line museum wishes to determine in which exhibit visitors are most interested among dinosaurs, anthropology and paleontology then each artifact may be encoded so as to identify the corresponding field (e.g. prehistory, anthropology, paleontology etc.) and a cookie stored on the computer of each on-line visitor may encode the relevant overlap data, as described above. 
     Order is of interest in the event that, for example, for a given campaign there are two dominantly visited websites, and the advertiser wishes to find out whether one of these websites is typically visited earlier, to the effect that by advertising only there the advertiser would gain substantially the same exposure and earlier in time. 
     Having described the principles of the invention, we will now describe various implementations it being understood that these are by way of example only. 
     Representation in the Cookie 
     The overlap server maintains in the cookie a list of the sites the user has visited per campaign. This list can be saved in one of the following ways: 
     Site ID List 
     The cookie holds a list of site IDs separated by a delimiter, allowing the sites to be in any order. 
     For example: site1;site2;site3. 
     Site Bit List 
     A sequence of bits represents the list of sites participating in a campaign. Each site is represented by a bit indicating if this site were visited, 1, or not, 0. The location of each bit in the sequence of bits is according to the ordinal number of the site in the database. Site 1 is bit  1 , Site 12 is bit  12  and so on. 
     For Example: 
     In a specific implementation, room for 40 sites is reserved in the cookie. A sequence of 40 bits represents a binary number between 0 and 2 40 -1. This binary number can be represented in base 62 as a 7 characters string with preceding zeros. 
     Defining the LSB as the first site makes the calculation easier. 
     For a campaign with 4 websites all of whose websites were visited the binary number will be, looking at the bits sequence from right to left, 000 . . . 0001111, which is 15. 15 in base 62 is F so in the string that will be written in the cookie is 000000F. 
     Overlap Server Database 
     The overlap server  14  needs to update the cookie with the sites that a user visited. In order to do this, each site in the campaign has to get a unique ordinal number. This number is accessed by the overlap server  14  and is used to determine what bit corresponds to a specific site participating in a campaign and, given a web user&#39;s exposure to a campaign, should be changed from 0 to 1. To this end, the unique ordinal number of each campaign may be stored in the database  15  or in any other file or database that is accessible to the overlap server  14 . 
     Table III below is built for a configuration embracing 3 sites: A, B, C 
     
       
         
               
               
               
             
               
               
             
           
               
                   
                 TABLE III 
               
               
                   
                   
               
               
                   
                 Combination 
                 Unique Users 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Only A 
               
               
                   
                 Only B 
               
               
                   
                 Only C 
               
               
                   
                 A + B 
               
               
                   
                 A + C 
               
               
                   
                 B + C 
               
               
                   
                 A + B + C 
               
               
                   
                   
               
             
          
         
       
     
     This table also facilitates calculation of the current reach and frequency, since summing only A with A+B and A+C and A+D and A+B+C and A+B+C+D gives the cumulative number of unique users who have visited website A since the start of the campaign. 
     If the order of the sites is important as well, a larger table is required as shown below in Table IV, which allows determination of how many users visited site A and then visited site B as opposed to users that first visited site B and then site A. For example for sites A,B,C: 
     
       
         
               
               
               
             
               
               
             
           
               
                   
                 TABLE IV 
               
               
                   
                   
               
               
                   
                 Combination 
                 Unique 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Only A 
               
               
                   
                 Only B 
               
               
                   
                 Only C 
               
               
                   
                 A + B 
               
               
                   
                 A + C 
               
               
                   
                 B + A 
               
               
                   
                 B + C 
               
               
                   
                 C + A 
               
               
                   
                 C + B 
               
               
                   
                 A + B + C 
               
               
                   
                 A + C + B 
               
               
                   
                 B + A + C 
               
               
                   
                 B + C + A 
               
               
                   
                 C + A + B 
               
               
                   
                 C + B + A 
               
               
                   
                   
               
             
          
         
       
     
     In order for the overlap server  14  to keep track of the order in which websites are visited not only must there be order-dependent records in the database  15 , but also the cookie must be configured to be order-sensitive. To this end, the cookie is configured to contain a sequence of data cells. Upon a web user&#39;s exposure to a campaign ad in a certain website, the overlap server writes an identifier of the site ID in the subsequent data cell. Thus if that user visited websites e.com, f.com, i.com in that order, the cookie will at that point display the following sequence: e.com identifier, f.com identifier, i.com identifier. The number of data cells required is the number of the websites participating in the campaign. 
     The potential maximum number of rows in the table for n sites will be: 
     
       
         
           
             
               
                 ∑ 
                 
                   k 
                   = 
                   1 
                 
                 
                   n 
                   - 
                   1 
                 
               
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                   n 
                   ! 
                 
                 
                   
                     ( 
                     
                       n 
                       - 
                       k 
                     
                     ) 
                   
                   ! 
                 
               
             
             + 
             
               n 
               ! 
             
           
         
       
     
     In accordance with one embodiment, the overlap server database contains fields only for the combinations/permutations that actually exist in reality, i.e., if no web users actually visited the combinations/permutations of, for example, websites BCD, then there would be no such line at all in the overlap server database for this combination/permutation. In such an embodiment the database structure is updated on the fly by adding new rows to the appropriate database tables as required. Alternatively, all possible relevant fields can be allocated in advance: although this will be wasteful of memory if it turns out that not all combinations/permutations were actually visited. 
     If the cookie holds the sites as bits then a table having the general structure of the table header shown below in Table V should exist: 
     
       
         
               
               
               
             
           
               
                 TABLE V 
               
               
                   
               
             
             
               
                 website bit 
                 website ID 
                 Campaign ID 
               
               
                   
               
             
          
         
       
     
     For each website that is added an increased ordinal number should be added that will specify its location in the list of websites. 
     Representation in the Log 
     In accordance with one embodiment, updating the database  15  is done by means of a log. A new entry to the log must be written for each new site that the user visited. The log contains the following information in each entry (line):
         Date   Campaign ID   The existing state—the list of sites the user has visited till now i.e. prior to the log being accessed   The updated state—the updated list of sites the user has visited. Actually this is the existing state plus the new site.       

     Implementation 
     A particular implementation of the algorithm will now be described. 
     The cookie stores the sites on which the user saw the campaign. The overlap server  14  checks the user&#39;s cookie and for the sake of explanation determines that the user already saw the campaign on sites A, B and C. In one embodiment of the invention, the overlap server  14  maintains a log of all changes that need to be made to the database  15 , but does not itself update the database. Instead, it conveys the log to a data warehouse (not shown) which maintains the integrity of the database  15 . If the user now sees the campaign on one of these sites, no unique user should be added to a new combination and the overlap server  14  therefore takes no further action. However if the user saw the campaign on site D, then the overlap server  14  marks in the log that:
         1 unique user should be removed from the combination A+B+C   1 unique user should be added to the combination A+B+C+D       

     The data warehouse receives the log and updates the database tables accordingly. Remote updating of the database  15  by the data warehouse offloads the need to do so from the overlap server  14  and avoids the need for the overlap server  14  to make repeated connections to the database. However, it will be appreciated that the invention also contemplates the possibility to update the database  15  by the overlap server  14 . 
       FIG. 4  is a block diagram showing functionality of the overlap server  14  in accordance with the first embodiment of the invention. As shown, the overlap server  14  comprises a memory  21  for storing a data structure containing records relating to each of the objects (S 1 , S 2  . . . S N ) and to each combination of two or more objects (S 1 S 2 , S 1 S 3 , . . . S 1 S N , S 2 S 3 , S 2 S 4 , . . . S 2 S N , S 1 S 2 S 3 , S 1 S 3 S 4 , . . . S 1 S N-1 S N  etc.). A visitor history determination unit  22  is coupled to the memory  21  for determining whether a visitor is visiting an object (S M ) for the first time. An object combination determination unit  23  is coupled to the memory  21  for determining a current combination of objects previously visited by the visitor. An incrementing unit  24  coupled to the object combination determination unit  23  and to the memory  21  and is responsive to the current combination being null for incrementing a respective record relating to said object (S M ). A decrementing unit  25  is coupled to the object combination determination unit  23  and to the memory  21  and is responsive to the current combination being non-null for decrementing the record relating to the current combination and incrementing the record relating to the combination of the object (S M ) with the current combination. An optional summation unit  26  is coupled to the memory  21  and adapted to calculate a total number of unique visitors to a specific object (S M ) by summing respective values stored in all records of the database  15  relating to the object (S M ). An optional output device  27  may be provided for outputting data subsisting in the data structure. Likewise, an optional tracking unit  28  allows for tracking changes in the visitor distribution and objects overlap over time, typically by comparing between two or more data states obtained from the data structure at different times. 
     Similar hardware is employed for implementing the second embodiment where order of visitation is important. The main differences are as follows. The data structure contains records relating to each of the objects (S 1 , S 2  . . . S N ) and to each permutation of two or more of objects (S 1 S 2 , S 2 S 1 , S 1 S 3 , S 3 S 1 , . . . S 1 S N , S N S 1 , S 2 S 3 , S 3 S 2 , S 2 S 4 , S 4 S 2 , . . . S 2 S N , S N S 2 , S 1 S 2 S 3 , S 1 S 3 S 2 , . . . S 1 S 3 S 4 , . . . S 1 S N-1 S N  etc.). The object combination determination unit  23  is adapted to determine a current permutation of objects previously visited by the visitor that conforms to a specified visitation order. The incrementing unit  24  is responsive to the current permutation being null for incrementing a respective record relating to the object (S M ); and the decrementing unit  25  is responsive to the current permutation being non-null for decrementing the record relating to the current permutation and incrementing the record relating to the permutation of the object (S M ) with the current permutation arranged in the specified visitation order. 
     In all cases, the data structure may be pre-compiled to contain all records in a complete set of records relating to all possible combinations or permutations. Alternatively, the incrementing unit  24  may be configured to create records in the data structure on-the-fly, possibly even starting from an initially empty data structure. 
     While a number of different embodiments have been described, it will be appreciated that modifications can be made without departing from the scope of the appended claims. For example, although the invention is principally applicable to determining the number of unique visitors to a site, it is also possible with minor modification to compute repetitions too. This could be achieved, for example, by allocating additional fields in the database, such as ABC and a separate line for ABCC. Such an approach would clearly require that the database tables be updated on the fly. 
     For example, the invention allows for real-time monitoring of visitor distribution and objects overlap. Whenever required, the then-current data subsisting in the data structure may be drawn and viewed in any one of a number of feasible implementations, including but not limited to a web-based reporting system, a reporting email, etc. 
     Further, as noted above, the invention allows for tracking of changes in the visitor distribution and objects overlap over time. By drawing then-current data from the data structure and comparing between two or more such data states obtained at different times, trends and other information may be derived from the comparison. 
     Likewise, while an embodiment has been described with particular reference to site reach in an on-line ad campaign, it will be appreciated that similar principles can be utilized in other applications. One example has been suggested above for use in an on-line museum to allow the curator to assess which artifacts or exhibits are most popular with the object of exhibiting more artifacts in those exhibits that are popular at the expense of those that are rarely visited. 
     However, the museum does not have to be an on-line museum since the same principles can be applied to a conventional exhibition. For example, an entry barrier to each exhibit can be equipped with a contactless reader and each visitor can be provided with a contactless tag having a unique ID, so that when each visitor visits a new exhibit his identity can be read by the corresponding reader. A database can be configured so as to store records for each combination of exhibits visited and updated in a manner analogous to that described above for on-line visits. 
     It will also be understood that the system according to the invention may be a suitably programmed computer. Likewise, the invention contemplates a computer program being readable by a computer for executing the method of the invention. The invention further contemplates a machine-readable memory tangibly embodying a program of instructions executable by the machine for executing the method of the invention. 
     In the method claims that follow, Roman numerals used to designate claim operations are provided for convenience only and do not dictate any particular order of performing the operations. 
     Finally, it should be noted that the word “comprising” as used throughout the appended claims is to be interpreted to mean “including but not limited to”.