Millisecond Forums

That's perfectly possible. You simply enter the trials in the desired proportions into the noreplace() selection pool in the <block>'s /trials attribute.

/ trials = [1 - 50 = noreplace(correct_pair, correct_pair, correct_pair, incorrect_pair, incorrect_pair)]

will run exactly 30 instances of <trial correct_pair> and 20 instances of <trial incorrect_pair> in random order, i.e. a 3:2 ratio.

I've done that, but is there any way to (for example) specify that within a block of 50 trials, the "correct pair" trial appears 30 times? Or is that not compatible with randomizing trials within blocks?

There isn't sufficient information here to give a response. Ordinarily, you simply create two <trial> elements -- one that displays a "correct pair" and thus requires a response, another one that doesn't -- and then you randomize their order via the <block>'s /trials attribute.