Repeating stimuli when using $ random selection in stimlist

[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.