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Blocking and looping stimulus randomly


I am setting up an experiment whereby I have 31 image stiulus and 9 video stimulus. I would like it to display 1 video stimulus after every 3 image stimulus at random. I have set up all of the excel files, loops and blocks as described in the faces/houses example; however I don’t know how I can set it to switch between stimulus. When I run the experiment, it shows all of the image stimulus before showing the video stimulus and so on. I only want the stimulus to all be shown once, but randomly and with videos between the images, making my experiment 40 trials long in total. How is this possible?



For a start, you need to describe your task much more precisely. Think what would be required for the methods section of a thesis, and then add a bit more.

For example at the moment, your description is internally inconsistent. If there are nine videos, being shown every three images, then there would be 27 images, not 31.

You also need to describe the randomisation of those stimuli, within images and videos and between images and videos.

Conceptually, is a “trial” one stimulus (whether image or video), or is it a sequence of three images and one video?

Step back for a moment and imagine being completely naïve to your experimental design. And then imagine being able to understand that design given nothing other than the text description above.

Part of the skill of describing a design is very precise, and hierarchical, terminology. Some of the difficulty here is that you use “condition” to mean different things in experimental design terminology. Be careful to distinguish conditions and blocks for example. At the moment you are conflating the two:

My experiment involves 3 conditions… The experimental condition has…
The task described is the control condition…

One could read this as saying that there are 3, 2, 5, or 6 conditions, although I suspect you mean that there are three different types of trials embedded within control and experimental blocks of trials. The term “condition” can apply either to the type of trial, or to the blocks, but not both.

Randomisation is an entire field of experimental design, and again, needs to be described precisely and hierarchically (at the level of stimuli, trials, conditions, blocks, subjects, and groups (if relevant)).

Happy to help, but it will be a waste of both our time trying to give suggestions with just this level of detail.