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Actually, I understand the question a bit differently. From what I gather, the goal is to ensure that each possible prime-target combination is realized (a combinatorial problem). So, prime #1 needs to precede each of the 8 positive targets and each of the 8 negative targets *exactly* once. Same goes for prime #2 and so on to prime # 16. This will yield 128 positive trials (16 primes * 8 positive targets) and 128 negative trials (16 primes * 8 negative targets). I think, the right way to pull this off is to use a set of 2(positive, negative) * 2(prime, target) counters for the item selection that contain the "hardcoded" permutations. For example, <counter counter_primes_positive> would list the item numbers for your 16 primes (8 times each because each prime needs to be combined with each positive target).
<counter counter_primes_positive> / select = noreplace / items = ( 1,1,1,1,1,1,1,1, 2,2,2,2,2,2,2,2, 3,3,3,3,3,3,3,3, 4,4,4,4,4,4,4,4, 5,5,5,5,5,5,5,5, 6,6,6,6,6,6,6,6, 7,7,7,7,7,7,7,7, 8,8,8,8,8,8,8,8, 9,9,9,9,9,9,9,9, 10,10,10,10,10,10,10,10, 11,11,11,11,11,11,11,11, 12,12,12,12,12,12,12,12, 13,13,13,13,13,13,13,13, 14,14,14,14,14,14,14,14, 15,15,15,15,15,15,15,15, 16,16,16,16,16,16,16,16) </counter>
The corresponding target counter would list the item numbers for the positive targets like this:
<counter counter_targets_positive> / select = current(counter_primes_positive) / items = ( 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8, 1,2,3,4,5,6,7,8) </counter>
With these two counters, each possible prime-target combination will be shown in your 128 positive trials. The same thing, of course, also needs to be done for the 128 negative trials. I've attached a small template script containing all necessary elements for further reference. My testing data show that the script performs as desired (each prime-target combination is run once in random order). Actually, this was fun!
Best, ~Dave
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