Timelines are not always lines: An evaluation of different timeline shapes
By: Sara Di Bartolomeo
Tags: Evaluation, Timeline, D3js, Data, Data Visualization

The value of starting simple

The data and tasks we ended up using, as well as the designs, are extremely simple. We argue that to fully understand the effect of the shape of a timeline, we have to start from the simplest datasets and tasks.

Here is an example of how a spiral timeline, with a mixed dataset (a schedule), could benefit the reader: the same hour can be at the same angle relative to the center for every round of the spiral, thus making it easier to find things that recur at the same time every day, while also making non-recurring events stand out. This is the design that we used in the experiment — the only difference being that in the actual experiment, the timelines contained 12 data points each.

Another reason behind the simplicity is that timelines are extremely widespread. Since almost everybody comes in contact with timelines on a daily basis, simple static visualizations play a valued role in communication.

As our target population was vast and very diverse, we decided that using Mechanical Turk to recruit participants was our best option, as it would allow us to reach a very large audience. We gave each participant a set of 12 questions, every time changing the dataset and the shape of the timeline we were showing them, so that they would observe each dataset type with each one of the timeline shapes.

What did we learn?

What we ended up finding out is that there is no overall winner, and no timeline is better than the others in every category. Some timelines work better than others in specific cases, while in some other cases, the choice of shape doesn’t make any difference.

For example, in terms of timing, in the what task — where participants were asked to find out the date in which a specific event happened — vertical linear timelines have performed better than all the other shapes for the mixed and the non-recurrent dataset. In the find task, instead, where participants were asked to find an event knowing both the date and the name of the event, the choice is mostly unimpactful.

The results are a bit more complicated than this, so we made a picture summarizing our findings in terms of timing:

Here is a summary of our findings about timing. A “<” sign means that the timing on the first shape is, overall, less than the second one, while a “=” sign means that there is no timing difference between the two shapes.

Here is a chart of all of the p-values for the timing resulting from the experiment.

More in-depth graphs and reports are available in our paper, at https://osf.io/2kdb9/

Regarding accuracy, instead, we didn’t find meaningful differences between the different shapes. Therefore, we can’t say that participants answered more precisely to questions on a shape rather than on another one. We just found out that the compare task, that was more memory-intensive than the others, produced worse results.

At the end of the experiment, participants could fill out an optional survey, in which they were asked to express their preferences and opinions.

We received some interesting comments that pertained directly to the shape of the timelines. This gave us a notion of how the participants felt about certain shapes to an extent that they cared enough to express it in an optional feedback section. In this case too the majority of the comments were in favor of the horizontal and vertical lines. The comments were of the similar trend:

“…the vertical line is easy to read”

“The horizontal and vertical graphs were easiest”

A few remarked in favor of the circular shape:

“…but circles were easiest for my eyes”.

The spiral shape also had a few admirers under its belt with comments like:

“The planting dataset with the spiral is a good example of a timeline that is functional and visually appealing”.

However, on the flip side, some of these participants revealed concerns about the spiral shape, stating it to be difficult and confusing. Their comments echoed similarly: