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Thread: Question re BGR and WGR values and effects (case 387)

  1. #1
    Join Date
    Aug 2007
    Posts
    13

    Question re BGR and WGR values and effects (case 387)

    At first I thought that BGR values (identified as "RBG" in the documentation, but as BGR in the samples) superseded the WGR values (likewise, identified as "RWG" in the documentation). That is, I thought that DirectRT would randomize blocks of trials using BGR values first, then use WGR values to randomize with the BGR defined blocks.

    But that simple rule doesn't appear to be the case. Consider this series of trials:

    block trial bgr wgr
    1 1 1 1
    1 2 1 2
    1 3 1 1
    1 4 1 2
    2 5 2 1
    2 6 2 2
    2 7 2 1
    2 8 2 2

    The sequence of trials doesn't follow the pattern I'd expected: I expected to get either trials 1-4 or 5-8, followed by the other four trials. That's not what happens.

    Could someone explain how DirectRT combines these two parameters to determine randomization order?

    Thank You!
    Last edited by jason_reed; 01-09-2015 at 12:38 PM.

  2. #2
    Join Date
    Nov 2005
    Posts
    3,318
    It's actually a bit more flexible than it would be if WGR values had to be kept within BGR blocks. In some cases, a user might want to randomize items from outside the blocks. For that reason, all the items sharing the same WGR values will be randomed with each other--i.e., we're talking about all of the items in the whole input file.

    In your illustration, you would be seeing trials from block 2 being mixed with those in block 1, because they share common WGR values. To avoid that result, you could use unique WGR values within each block. That would prevent any crossover.

    e.g., try it like this and see if you get the desired result.

    block trial bgr wgr
    1 1 1 1
    1 2 1 1
    1 3 1 1
    1 4 1 1
    2 5 2 2
    2 6 2 2
    2 7 2 2
    2 8 2 2

    With regard to the naming convention (WGR/BGR vs RWG/RBG), a quick search on the forums seems to clearly indicate that we have standardized with the use of WGR/BGR. I will adjust the documentation accordingly. Thanks for pointing that out. I think the same thing applies to MediaLab.

  3. #3
    Join Date
    Jun 2008
    Posts
    46
    Quote Originally Posted by jarvis24 View Post
    With regard to the naming convention (WGR/BGR vs RWG/RBG), a quick search on the forums seems to clearly indicate that we have standardized with the use of WGR/BGR. I will adjust the documentation accordingly. Thanks for pointing that out. I think the same thing applies to MediaLab.
    FYI, I found this thread after being confused by the tutorial documentation that came with DRT 2012, which still refers to RWG/RBG.

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