Hot to make RWG with the constraint of equal numbers of stimuli of each type?

Hi all,

Apologies for the naive question, but...

I'm able to do a RWG (randomization within groups) for my stimuli using different rwg values (I have four different sentence types, so I have about 30 groups of four stimuli having the same rwg value), no problem there.

however, I need an additional constraint: I need for each subject to see an equal number of each of the four different types of stimuli (I don't want my subjects to happen to have, say, 5 stimuli of type A, 10 of type B, 3 of type C, and 12 of type D or anything like that!

So, I don't know if we require a metafunction here, or if it's possible at all to have such constraint.

Any ideas would be much appreciated!

Giorgos

You could take the approach of assigning putting the 30 stimuli from each group into four different stim lists. Say list1.txt, list2, list3 and list4. Then randomly draw from them (without replacement) in 30 blocks of 4 trials like this:

$list1 (wgr=1)
$list2 (wgr=1)
$list3 (wgr=1)
$list4 (wgr=1)
$list1 (wgr=2)
$list2 (wgr=2)
$list3 (wgr=2)
$list4 (wgr=2)
...
$list1 (wgr=30)
$list2 (wgr=30)
$list3 (wgr=30)
$list4 (wgr=30)

Or some variation on that. Does that make sense?

Thanks very much, I think it does, will try it and let you know,

Cheers!

Giorgos

Hi there, sorry to bother you again, but I seem to have mislead you with the description of the stimuli: You see, I have 30 stimuli of four different versions each, but my constraint is not that I have to have every single version of each of the 30 stimuli occurring, but, quite on the contrary actually, that:

- only one version (a,b,c,d) out of four is selected out of each of the 30 stimuli
- in the end, I should have an equal number of a's with b's, c's, and d's.

What you have suggested that I should do makes the all four versions of each of the 30 stimuli appear, that's why I'm saying I wasn't clear enough (profuse apologies for that).

Thanks a mill for your patience and time

Giorgos

Not sure what you mean by:

[QUOTE]
one version (a,b,c,d) out of four is selected out of each of the 30 stimuli

[/QUOTE]

Do you mean that you have 30 a's, 30 b's, 30 c's and 30 d's? And that you want to pick one from each so that you have a set of 4? And then that you want to do this over and over again (10 times? 30 times?) so that the subject is given multiple sets like this? And is the [I]order[/I] within each chosen set also to be random (e.g., adcb, bcad, etc.)?

Hi,

thanks for your time on this.

So, basically, I have:

1a, 1b, 1c, 1d
2a, 2b, 2c, 2d
....................
40a, 40b, 40c, 40d

Now, for each number (i.e., 1, 2, etc) I want for each session to randomly pick out only one version (a,b,c,d) for each number. So, if 1a appears, I don't want any 1b,1c, or 1d appear. Plus, in the end, I want to have for each session 10 a's, 10 b's 10 c's and 10 d's. But no stimulus appearing in more than one version in one session either. Does that help at all?

Thanks so much

Giorgos

So, yes, the order would ideally differ as well, I don't need it to be fixed at all.

Cheers

G.

And ideally I want each subject to see a total of 40 stimuli, different version (b instead of a), and different order (stimulus "3" in the place of stimulus "1") for each subject....

Cheers

G.

So you want to control the pattern of randomization [I]across subjects?[/I] So, e.g., if you have 100 subjects, then 25 will have seen 1a, 25 will have seen 1b, 25 will have seen 1c and 25 will have seen 1d?

Hello, thanks for your response. Yes, that's right, sorry for not mentioning that.

Cheers

G.

Ahh, ok, then the answer is easy--no, there is no way to do that automatically with DirectRT. Each session is independent. Between subject factors need to be dealt with via different input files.

Cheers, sorry for all the fuss

G