Implementing a task, trial and stimulus in Tobii Pro Lab

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...

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...

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.   

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:

• Structuring the stimulus presentation and dealing with order effects in Pro Lab
• Between- and within-subject experimental designs in Pro Lab
• What are experimental variables?
• Independent Variables and Participant Groups