Random selection of stimuli from same pool without replacement


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davidbarton
davidbarton
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Posts: 11, Visits: 60
Hi, please can I get some help with a problem I am experiencing.

I have a single pool of 60 faces which need to appear in pairs without replacement across 30 trials. Faces appear next to each other onscreen. I am having trouble figuring out how to get each face only to appear once as they are appearing randomly on both the right and left side of the screen. As the faces get pulled from the item list they should them be excluded from further use. It makes no difference if they appear left or right as long as its random and only once. But once they appear they should not appear again.

Any help will be appreciated. Many thanks

David Barton

<picture faceleft>
/ items = faces
/ select = noreplace
/ size = (45%, 45%)
/ position = (30,45)
</picture>


<picture faceright>
/ items = faces
/ select = noreplace
/ size = (45%, 45%)
/ position = (65,45)
</picture>

<item faces>
/1="face (1).jpg"
. . . . .

/60= "face (60).jpg

</item>


<trial facetrial>
/ validresponse = ("z", "/")
/ correctresponse = ("z", "/")
/ stimulusframes = [1=faceleft,faceright]
</trial>

<block Election>
/ trials = [1-30 = noreplace(facetrial)]
/ errormessage = true(error,200)
/responsemode=correct
/ preinstructions = (instructions)
/ recorddata = true
</block>




Dave
Dave
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Posts: 12K, Visits: 100K
This isn't a single selection pool of items. Every <picture> element has its own distinct selection pool of 60 items given the way you set this up, i.e. there are *two* independent pools -- the fact that both reference the same <item> element does not change this. If you want the two <picture> elements to sample from the same, single selection pool, you need to use a <counter> element:

<picture faceleft>
[...]
/ select = mycounter
</picture>

<picture faceright>
[...]
/ select = mycounter
</picture>

with

<counter mycounter>
/ select = noreplace(1-60)
/ selectionrate = always
</counter>

Edit note: You also need to set /selectionrate appropriately in the use case at hand.

Edited 10 Years Ago by Dave
davidbarton
davidbarton
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Group: Forum Members
Posts: 11, Visits: 60
Works perfectly Dave, thanks heaps.

Best Regards

David

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