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Thread: randomizing within a fixed alternating pattern

  1. #1
    Join Date
    Apr 2007
    Posts
    13

    randomizing within a fixed alternating pattern

    hi,
    in my experiment i have 2 types of trials, A-trials and B-trials. both trial types should all be randomized within each other (i.e they are all treated as one group for wgr). BUT i also need to make sure that B type trials do not follow each other. if a B trial was presented, the next trial must be of type A. is it possible to implement this?
    thanks!

  2. #2
    Join Date
    Nov 2005
    Posts
    3,328
    So it should always be ABABABABABA... etc? If so, just assign all the A's a WGR of 1 and all the B's a WGR of 2. Yes?

  3. #3
    Join Date
    Apr 2007
    Posts
    13

    no

    the proportion is 4 A-type trials to 1 B-type trial. so it can be AAAAAAABABA etc.

  4. #4
    Join Date
    Nov 2005
    Posts
    3,328
    I've been working on a nice simple notation system for creating constraints like that, but I haven't programmed it. Might be working by the next version. The only way I can think of to do this in v2006 would be to treat the BA's as BGR pairs and draw the stimuli randomly from stimlists as follows:
    Code:
    BGR WGR STIM
    1   0   $A
    2   0   $A
    3   0   $A
    4   0   $A
    5   0   $A
    6   0   $A
    7   0   $B
    7   0   $A
    8   0   $B
    8   0   $A
    Last edited by jarvis24; 04-11-2007 at 11:24 AM.

  5. #5
    Join Date
    Apr 2007
    Posts
    13

    my solution

    because i'm using compound screens, drawing the stimuli from a stimlist wouldn't work in my case.
    after some thinking & consulting, i implemented the following solution:

    since there are exactly 4 different A-type stimuli and only one B-type stimulus, i created 4 compound B trials: BA1, BA2, BA3, BA4. then i randomized these trials with the regular A trials:
    WGR stim
    1 A1
    1 A2
    1 A3
    1 A4
    1 BA1
    1 A1
    1 A2
    1 A3
    1 A4
    1 BA2

    and so on.
    so, i've ensured that after each B trial there would be an A trial, but the stimulus for the particular A trial is randomized.

    i hope this is clear.
    michal.

  6. #6
    Join Date
    Nov 2005
    Posts
    3,328
    So you stuck two trials together into one? i.e., whenever you have a B trial, you automatically get an A trial because it's actually stuck onto the end of that B trial?

  7. #7
    Join Date
    Apr 2007
    Posts
    13

    yup

    that's exactly what i did. what do you think?

  8. #8
    Join Date
    Nov 2005
    Posts
    3,328
    Clever!

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