So, thank you all for your kind answers.
To summarize, in my case I have 2 predefined lists: one containing the items and another containing the possible combinations. My list of items is something like:
Item 1
Item 2
Item 3
Item 4
My combinations list has the combinations of items possible. It goes like this:
1 2
1 3
1 4
2 3
2 4
3 4
So, I created a general "RandomQuestions" constructed list. If I want to display 4 questions in my questionnaire, it would be like this:
Begin Unverified Perl
my $number_questions = 4;
ADD("Combinations");
RANDOMIZE();
LISTMAX($number_questions);
End Unverified
Then, for each question I created another constructed list. For instance, for question 1, it is:
Begin Unverified Perl
my $tmpString = "";
my @tmpItems = ();
$tmpString = LISTLABEL("RandomQuestions", 1); # in the 2nd question it would be LISTLABEL("RandomQuestions", 2), etc
@tmpItems = split(" ", $tmpString);
ADD("listItems",$tmpItems[0]);
ADD("listItems",$tmpItems[1]);
RANDOMIZE();
End Unverified
Just a final question: although I think this is correctly working, does anybody know a way of debugging this, other than running the questionnaire several times and empirically see if the combinations are being repeated?
Thank you all again!