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Randomization without replacement using images

  • 24 June 2020
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Hi all,
I am trying to run an experiment with 3 conditions in a within-subjects design. I have 25 pairs of images which I want to randomize without replacement for each block within the survey. For example, when Pair1 appears in Block 1, it won't reappear in Block 2; and if Pair1, Pair2 appear in Block1, Block2 respectively, Pair3 will appear in Block3.
Basically, I want the selection of image pairs to be fully randomized (so the order could also be: Pair1 appears only in Block2, Pair2 appears only in Block3, Pair3 appears only in Block1). Once a pair of images appears, I do not want it to appear in any of the future blocks. Also, I want the blocks to be randomly presented.
My survey has 13 questions in total that are distributed across 3 blocks (i.e., the conditions in my experiment). I have tried to embed the images in the survey flow such that each block has the same pair of images within the block but when I move on to adding if-then branches before the next block, I am not sure what I need to do to make this design possible. I have the survey flow set up in this way (screenshots attached) but by the time I get to the third block (PTTest1) I am confused about how to set up the if-then branches.
Any guidance would be appreciated!
Screenshot 2020-06-24 at 5.37.07 PM.pngScreenshot 2020-06-24 at 5.38.01 PM.pngScreenshot 2020-06-24 at 5.38.55 PM.png

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Best answer by MaiaH 27 July 2020, 17:34

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MaiaH
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Hi solanki7, it sounds like you may be able to accomplish this either with complex branch logic, or you could randomly assign participants their 3 pairs at the start and then display these assignments as unique blocks in the survey.
This is largely because having 25 groups for each pair and choosing 3 has a large number of permutations. Instead, if all 3 blocks are the same in terms of content, we can set up a single randomizer with 25 groups nested within.
Then we can create 25 identical blocks to correspond with each pair that will be nested within the groups of the randomizer but will differ in which image pair they will display. The randomizer then only needs to randomly display 3 items from the nested groups to ensure that items are both randomized and displayed without replacement!
If you have additional questions about setting this up or troubleshooting your original setup, please feel free to contact our Support Team for further assistance! 🙂

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