How to repeat letter presented two steps before?

Hello, I'm having truble to create an experiment with the next design: 100 trials: each trial presents one letter in the middle of the screen (out of 10 possibles ones (A-J)), when 20 trials need to presents a letter showed 2 step before him (two-back condition), and 80 more trials presents a random letter in a random order (except the letter showed two letters before him)... did someone have an idea??

thanks :)

Answer 1

[Jos (10584)]

This seems to be some kind of homework assignment (which is fine!) so I will just give you some thoughts:

1. You have a set X of letters
2. Randomly select a letter L from X (using e.g. randperm)
3. Put that letter L at position k in the sequence of letters S
4. Remove L from X
5. Get the letter at position S(k-2) and put that back into X
6. Return to 1 until you filled the whole array

Now none of the letters in S are targets, so to create targets:

1. select 20 random positions i (using randperm)
2. Set the letters at positions i+2 to the letters at position i (S(i+2) = S(i))