Structuring the stimulus presentation and dealing with order effects in Pro Lab

Tobii Pro Lab Design Screen based projects

Learn how to plan and structure your stimulus presentation order in Pro Lab

Structuring your experiment using shuffle, repetitions, sampling, and randomization

Depending on the experiment and the type of stimuli, different presentation orders are relevant. The simplest possible form would be to simply show all the stimuli in whatever order they have been put in the group, from the first to the last. Such a sequential order would be very appropriate if you have a group of images containing instructions for the participants. Obviously, they have a reading order and should not come randomly.

For other stimuli, like experimentally manipulated images, a random order makes the most sense, for the aforementioned reasons of order effects. In many experiments, however, the trial does not consist of just a group of images. You may have a fixation cross in the center of the screen to start off the participants in a search task from the same point. In this experiment, we do not want to just randomize the presentation order of everything, so we end up with several images or fixation crosses in a row. Instead, we want to have the fixation cross fixed at the beginning of every trial, and then just sample one image from a group of images, one at a time.