do not want same stimulus in a row but need overall random selection

[David McFarlane]

Sarah,

This topic comes up again and again, as you can see at 
http://www.pstnet.com/forum/Topic2186-5-1.aspx , 
http://support.pstnet.com/forum/Topic3166-5-1.aspx , and 
http://support.pstnet.com/forum/Topic3178-5-1.aspx , or by running 
your own search with appropriate search terms (I like to refer to 
"consectutive repeats" or "tandem repeats", but some folks refer to 
"runs" or "two in a row" or "twice in a row").

You have a very simple case, so you might make do by adapting the 
NoRepeat.es example downloadable from PST.  Just know that their 
algorithm will fail for cases of any complexity, as it is a form of 
"bogosort" (look that up on Wikipedia).  By contrast, you can find an 
elegant, formally proven, general solution in Remillard, G. (2008) "A 
program for generating randomized simple and context-sensitive 
sequences", Beh Res Meth 40 (2): 484-492, and Remillard, G., & Clark, 
J. M. (1999) "Generating fixed-length sequences statisfying any given 
nth-order transition probability matrix", Beh Res Meth, Instr, & 
Computers 31: 235-243, but I have not seen this implemented yet in 
E-Prime.  I have a less elegant but possibly more serviceable 
algorithm that I present briefly in the above threads, although I 
still have not gotten around to implementing that in E-Prime myself 
(I did do it long ago in C, which I still find a much more suitable 
platform for this kind of work).

Good luck,
-- David McFarlane, Professional Faultfinder
"When all is said and told, the 'naturalness' with which we use our 
native tongues boils down to the ease with which we can use them for 
making statements the nonsense of which is not obvious."  -- Edsger 
W. Dijkstra, "On the foolishness of 'natural language programming'" 
(http://www.cs.utexas.edu/users/EWD/transcriptions/EWD06xx/EWD667.html)


At 1/26/2010 12:24 PM Tuesday, Sarah wrote:
>I am making a simple stroop task with 4 colors, and about 20 trials (1
>trial list and 1 trial procedure). I have already selected Random
>Selection under the property pages for my trial list, but how do I
>ensure that a red target won't come after another red target, or a
>blue after a blue, etc? Is an inline necessary?
>
>Thank you

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