Randomizing stimuli with restriction

psych_student

Hi guys!

Thank you for this amazing software and the great help on this forum!

I have a question regarding randomization. So I have 14 images from a movie the participant saw earlier and 14 images that are similar but not appeared in the movie. In each trial I would like to present simultaneously a screenshot from the movie and a similar image of a scene which wasn't present in the movie. The task for the participant is to select the screenshot which he thinks he saw earlier.

I'm familiar with the randomization technique of putting all the combinations in a loop and working with coördinates, but in that case each stimulus pair would be shown twice (filmscene1 on the left and the fake on the right et vice versa), I would like to make it so that each stimulus pair appears only once.

I also want to include 2 buttons, in that way the participant can make a choice between left and right, not really sure if you can do this all in the sketchpad or you need to create a new canvas for that?

Thank you in advance.
Kind regards,

Benjamin

eduard

Hi Benjamin,

Are the film images always paired with the same fake images? If not, I can't really see how the standard randomization inside a loop table would work. But you say, you figured that part already out?

I also want to include 2 buttons

Add a keyboard_response item to your sequence and set the allowed/correct responses to the keys that you want to be used. If you want to use a responsebox, a mouse, or something else, the response collection will look a bit different of course.

Hope this helped.

Eduard

psych_student

Hi Edouard,

Thank you for your response!
I figured out how to randomize stimuli with the loop table, but if I do that each stimulus appears twice (1x film - fake & 1x fake - film) and I don't want that to happen.

So, indeed: for each film image I have a paired fake image.
If I put everything in the loop table I got 28 trials (14x film image left & fake image right et vice versa) and I only want 14 trials, so 14 combinations should be 'skipped' in that case.
Any idea how I could implement that?

For the 2 buttons:
My first idea was to show clickable buttons under the stimuli on the screen, but indeed, a keyboard_response would be good too

Kind regards,
Benjamin

eduard

How did you implement it? Well, the easiest solution would probably be deleting all the lines in the loop table don't you need, but you probably want also some counterbalancing, so a variable that makes sure that a fake image is just as often on the left as on the right side, don't you?
Not sure whether it is that easy, it depends on your implementation.

Eduard

psych_student

So I made this table but in this case I have 28 trials and I do only want to show the participant 14 trials. Counterbalancing is not needed here because I need this task for a follow-up session, it's more of a check if they paid attention to the movie earlier.

So I only want to show 1_film on the left and 1_fake on the right OR vice versa, so not both, and so on. And this in random order, in that way that the film image appears sometimes on the left, sometimes on the right, but not in a systematic way.

Kind regards,
Benjamin

eduard

Why don't you just delete all duplicates, if you don't care for counterbalancing?

psych_student

I was looking for a random way to do it but indeed, if you don't counterbalance within or between subjects, than it doesn't matter whether it's a fixed sequence or not.

Thanks for your help!