Randimization without replacement in blocks

[David K McFarlane]

Noga, 


Just to make sure I understand, I'll put some example numbers on this.  You 
have, say, 30 pictures, and you want to present 10 pictures each in 3 
blocks, and you want each picture to appear only once across the entire 
session.  If that is the case, then this is probably a job for nested lists. 
Put all 30 pictures in a nested list that will be used by all three blocks.  
Set the nested list to Random selection, and make sure that "Reset on every 
run" is *off*.  Now the 30 pictures will be randomized once, and each block 
will take samples from that one random order. 


On the other hand, if you did want to allow pictures to appear only once 
within a block but allow them to reappear across blocks, then turn on "Reset 
on every run" for the nested list. 


 -- David McFarlane 


> My experiment contains 3 blocks with break between them. on each block
> I present pictures from a list. I want each picture to appear only
> once! and in random order. My problem is that pictures can appear more
> than once (the same picture can appear in block 1 and in block 2 or
> 3). Pictures that appeared in block 1 are not excluded from appearing
> again in the next blocks.
> How can I fix it in an elegant way? without separating the pictures
> list to 3 lists (list for each block)? 
> 
> Thanks! 
> 
> Noga



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