Repeating stimuli when using $ random selection in stimlist

[jarvis24]

[edited from support email]

We tried to use the $ random without replacement with a StimList. It seems though that the program does repeat an item from a stimList BEFORE it uses all the items in the list. We expect a single appearance of each item from the list when we call the list 8 times and it has 8 items. Similarly, when we call the list 6 times, we expect 6 different items. Do you have a suggestion why this occurs?

[jarvis24]

A repeated stimulus will usually only occur once the whole list has been used. Depending on your input file structure (e.g., a randomized block of 8 trials that draws from a stimlist containing 6 stimuli), it may appear that this is not the case, but it's a very reliable function. If you send along your input file, I'll take a look at it.

[barilan]

I appreaciate your help.

[jarvis24]

In DirectRT, the random selection of items through stimlists occurs before the trials themselves are randomized through BGR and WGR.

Your trials occur in pairs--and each pair of trials is accessing the list 8 times. But there are 12 stimuli in the list--so look what happens... The first pair uses 8. The second pair uses the 4 that are left over and then draws 4 more from a fresh set of 12. This makes it very likely that the second set will have repeated stimuli. Because you are also randomizing the order of the three sets, you could see this repetition occur at any point--depending on where the second set ends up being presented.

Does that make sense?

An easy fix would be to place just 8 stimuli in your stimlist instead of 12. Or you could create a separate stimlist with 12 stim for each pair of trials. That way they can draw 8 (or any number) and it won't affect any other trial sets. The lists could even be identical so long as they have different names.

Hope that helps,
Blair

[barilan]

Thanks for the help.

I thought the list restarts itself in each trial...isn't there a function which does that?

Tali

[jarvis24]

Not that I know of--if the documentation implies that anywhere, please let me know. The only way to do that would be to create a separate stimlist for each trial.

[jarvis24]

[Edited from related support email]

There seems to be something wrong with the $ randomization without replacement function in our DirectRT v2006. In an IAT input file, we specified randomization without replacement for stimlists of white and black faces as well as pleasant and unpleasant words. To make sure that the program draws from a fresh list of these stimuli for each block of trials (and thus prevent any carryovers), we created new stimlists for each block of trials and gave them different names, although they each contain identical stimuli (so the stimlist "white" contains the same stimuli as "white_a" and "white_b"). However, within each block of trials during the IAT, items from one stimlist will repeat before all the items in the stimlist has been presented. For example, the same white face, or pleasant word, will be shown twice (sometimes even sequentially) before a different white face or pleasant word has been shown. Do you have any idea why this might occur? I've attached the IAT input file.

I think so. If we use Block 3 as an example, note that you draw five times from the list "white_a" yet there are only four items in the list. Consequently, DirectRT must refresh the list and pick from it again for the fifth stimulus. This will necessarily result in one stimulus being used twice. Because the randomization of trials occurs after the selection of items from stimlists (see above), it is possible then to see the repeat stimulus as early as the second in this particular group.

Take a look at the IAT sample that comes with DirectRT in the Samples folder and see if the assignment of stimuli makes sense. Let me know if you have any trouble with it.

[nilida1]

Hey
I'm using a $ stimlist and I want to repeat a stimuli twice in the sme trial.
How do I make the software repeat a stimuli without knowing in advance which one it will pull from the stimlist?

thank you
Nili