Implementing a task, trial and stimulus in Tobii Pro Lab

Tobii Pro Lab Screen based projects Screen Based Eye Trackers

Learn how to implement different experimental design components in Tobii Pro Lab 

Once you are ready to implement your experiment, you will use the Design module in Tobii Pro Lab to setup your different experiment elements. In Screen based projects this is typically done by creating one or multiple timelines and adding different design elements on the timeline. 

Before you continue, we are assuming that you have already defined the different details of your experimental design, such as:

  • The experimental variables (independent, dependent and extraneous variables)
  • Participant design (Within- or Between participant design)
  • Tasks, trial structure and stimulus design
  • Supporting elements (for a definition see the table below)

If you sort out these details in advance, it will not only allow you to implement your experiment efficiently in Pro Lab but also account for the data analysis and export in your implementation. 

The table below, shows you a list of common terms used in experimental design. 

 Table 1. Common terms of an experiment

Term

Definition

Task In its simplest form a task can be understood as “what subjects have to do in an experiment” (Philipp & Koch 2010), or it can be also defined as the way participants are asked to respond to stimuli in an experiment. An experiment can contain single or multiple tasks. Tasks are composed of different elements such as instructions, trials, and supporting elements.
Trials In most experiments, a trial refers to a set of stimuli and supporting elements, that most often form self-repeating blocks of an experiment.
Stimulus The stimulus represents your independent variable, and its possible variations. A stimulus is something the researcher presents to the study participants and simultaneously measures their response to it.  
Response A physical or physiological change that can be measured. Some examples are pressing a button, saying a word, moving the eyes, pupils dilating, heart rate increase, etc. The response is related to your dependent variable.
Supporting elements Supporting elements are all other elements in the experiment that you present to the participant but do not measure their responses, i.e. these elements are not necessary for computing your dependent variable. For example, cross marks used for centering the gaze of a participant before the start of the stimulus presentation, instructions, fillers, etc.

Reference:  Philipp, A. M., & Koch, I. (2010). The integration of task-set components into cognitive task representations. Psychologica Belgica, 50, 383–411.

In order to understand the relationship between these different terms and how experiments are implemented in Pro Lab’s Design module, let’s use a practical example.

 

 

 

Testing two versions of an ad

Imagine that you want to run a study to evaluate the impact of two versions of the same ad on the viewing behavior of a group of consumers. In particular, you want to know if a specific ad item, that varies between the two versions, distracts the viewers from reading the text that contains the main message.
Since the two ads differ only in that item, they will appear quite similar to the participants. Consequently, to avoid any carry-over effects caused by the exposure to the two versions to the same participant, you decide to use a between-subject design. 

This research question can be operationalized by using a simple free viewing task, where you provide an instruction to the participants to simply look at the different ads and measure their visual behavior towards the two versions of the target ad.

To make your research question less obvious to the participants you decide to include several filler ads mixed with the target ads (stimuli). You also decide to use a fixation cross image before each ad (fillers and targets) to standardize the start point of the viewing behavior of each participant. Each pair of fixation cross plus ad (fillers and targets) will constitute your trials. Finally, since you opted for a between-subject design you create 2 versions of the same task, each version containing the same filler ads but a different target ad.

This is how your planning could look like...

Experiment task scketch

Before we start the practical work of implementing it, let's summarize the design:

  • You want to implement an experiment with two tasks that only differ in one element, the target ad.
  • Each task will be presented to a different group of participants.
  • The tasks start with and instruction followed by the presentation of trials containing an ad (fillers or target) preceded by a fixation cross.

Until now we have been using common experimental design terms (Table 1) decribe the experiment, the following steps will show you how to operationalize the terms in Pro Lab.

  1. First, create a new Screen-based project and click Design to open the Design module.
  2. Let' start by building the first task. On the existing Timeline, add an Instruction element.
  3. Double-click the Instruction element to open the Instruction editor window.
  4. Type and format your instruction text, then click Save when you are done.
  5. In the Properties panel, on the right, set up how long the participant views the instruction and how to advance to the next stimulus. Choose between advancing automatically based on a preset Time, advance by pressing a key (Key press) or by performing a Mouse click
  6. Next add a Group element to the timeline. This element will contain your trials elements and manage their presentation order. 
  7. Your experiment requires that the four fillers and one target ad are presented during the task. In the group Properties panel, located to the right, set the Action of the group to Repeat and the number of Repetitions to 5. to present a total of 5 trials during the task. Name the group Trials.
  8. Double-click the Trials group element to open the group Timeline, i.e. the trial timeline.
  9. Your basic trial structure is composed of a fixation cross followed by a filler or target ad. The fixation cross is an unchangeable element while the ads will change with each trial. So to implement the trial structure you need to use an unchangeable element and a variable element. 
  10. Start by adding the unchangeable element first, in this case an Instruction element.  An Instruction element is a quick solution to generate a fixation cross.
  11. After you added the Instruction element, find the Properties panel, on the right, then change the element's Name to Fixation cross and set it to advance automatically after 3 seconds (select Time then set it to 0 min and 3 s).
  12. Double-click the Instruction element. Change the Text font size to 72 and type in a plus sign (+). Click Save when you are done.
  13. The next element on the trial timeline will be a variable element - a Group element. This element will work both as a container for the fillers and target ads, and control the sampling order of each of the images.
  14. After adding the Group element to the trial timeline, find the Properties panel, located to the right, set the Action of the group to Sample and Presentation order to Random without replacement. This will randomize the presentation of the ads, without presenting a particular ad twice. Set the Sample size to 1. Name the group Ad sampler.
  15. Double click the Ad sampler Group element to open the group timeline, i.e. the ad container.
  16. Add stimuli in Pro Lab by drag and drop the four fillers and target ad files to the group timeline. So you will start building the trial structure by adding a new Instruction element, which you will use to present a fixation cross to the test participant.
  17. In the Properties panel, on the right, set the Image elements to advance automatically after 7 seconds (select Time then set it to 0 min and 7 s).
  18. You are done designing the first task. To implemement the second task, simply create a second timeline by clicking the Duplicate timeline icon, that appears when you hoover the mouse over the timeline name on the Timelines panel on the right .
  19. Replace the target ad version by the other version by opening the Ad sampler group of the new task, deleting the existing target ad element and drag and drop the image file of the other version of the ad to the group's timeline.
  20. Name your timelines in a way that you know which version of the ad the timeline contains, e.g. Ad A and Ad B. This will help you to identify which timeline to use during the recording phase. In this case the timeline name will also be used to segment the data according to the independent variable during the analysis.

When you are finished, your experiment implementation should look something like this...

Ad AB test Pro Lab

Let's summarize the implementation, now from Pro Lab's Design module perspective:

  • You used a Screen-based Project to define your type of experiment and host it's different components and data.
  • You used two Timelines to create two distinct tasks and to set up the order of the different task elements.
  • You used two Instruction elements, one to instruct the participant what to at the start of the test, and the second one to create a fixation cross stimulus to center the participant's gaze at the start of each trial.
  • You used two Group elements, one to structure your trials, and the second one to host your ad images (fillers and target).
  • Six images were added to one of the Group elements, creating six Image elements. Two of them are the stimuli where you will measure the participant response - the target ads, and the other four are simply fillers or supporting elements.  

During the course of this example you have learned how to implement and operationalize an experiment in Pro Lab. The current example is fairly simple in it's implementation and did not cover all the available design elements (Table 2) or challenges. But we hope it gives you a basis to understand the approach you need to use to translate your own experiment into Pro Lab.   

Table 2. Pro Lab's experiment and task design terminology.

Terminology

Function

Project The Project is the main directory for your study. It contains all the stimuli, your task-trial implementation, recordings, event data, participant information and your data selection definitions for analysis (times and areas of interest). There are different types of projects, depending on your study setup and type of eye tracker used to record eye movement data.
Timeline Set up the task and its elements in "chronological" order. You may create different Timelines within your Project to help you manage your study design.
Element It is an object you add to the timeline. This object will either work as a container for other elements or present and generate media. Each element posesses properties that you can change to customize the presentation of the content (according to the type of element). 
 Add "stimuli" (add images and video) This element generates image and video elements and stimuli, using a single media file. The suported media files are  .bmp, .jpg or .png for images, and, .gif, .avi or .mp4 for video. Use external tools such as the Adobe Creative Suite, Elements or other free tools such as Gimp and  Inkscape, to create the images and video. Here the word stimulus does not have the same definition as Table 1, as these elements can be used to present supporting elements as well.
Add Group This element works as a container for other elements and has it's own timeline. It is used to add rules and actions to your presentation (e.g. to randomize and repeat the presentation of a group of images).
Add Web This element will launch a URL in a Pro Lab Browser window. It automatically generates video recordings of the viewport, full page screenshots, web navigation TOIs and associated events. Using this element makes it easier to analyze and aggregate data of browsing behavior.
Add Screen recording This element allows you to capture attention for any other type of stimuli shown onscreen (e.g. non-supported browser webpages, non-supported video, other software). It generates video recordings of what is shown on the screen.
Add Instruction This element enables you to include study introductions, directions, and/or task information as text on the screen. Using these embedded text instructions instead of verbal prompts helps to ensure that the testing procedure is consistent across all participants. These elements won’t be automatically available in the analysis tools (metrics and visualizations).

 

 

Conclusion

Pro Lab is a tool that allows you to creatively build and configure different types of eye tracking experiments. However, like other experiment building software, it has a certain logic and uses certain terms that require you to learn, before it can be used efficiently. This article explores the relationship between common research terms and the terminology used in Pro Lab and aims to help implement your design in Pro Lab.

For more terms and examples read also:

 

 

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