Randomize a STIMULUS withing a group (case 6149)

[acwhite327]

In my task I have 32 trials (16 each of 2 different stimuli) and I want to randomly have a tone play on 8 trials (4 for each stimulus; so it's not really random). Is there a way to have a random stimulus that is assured to appear on only 25% of the trials within a block? Thanks!

[jsimons]

I think the simplest way to achieve what you describe would be to simply add the tone to eight of the trials in the input file. Then, you can use within-group randomization to present the trials in a random order. That way, the tone will play for 25% of the trials, four times for each stimulus, but at a random position in the series of trials.

So, to give a smaller example, you could set up something like the following (and then randomize the presentation order);

1: Stim A, no tone
2: Stim A, no tone
3: Stim A, no tone
4: Stim A, tone
5: Stim B, no tone
6: Stim B, no tone
7: Stim B, no tone
8: Stim B, tone

Would that achieve what you wanted?

[acwhite327]

thanks! That sounds pretty much exactly like what I did and it all worked.