The whales and dinosaurs variables are numeric (Long) variables that record the rating, on a scale of 1-5, that respondents gave to the Whale and Dinosaur galleries respectively. The expression subtracts the rating given to the Dinosaur gallery from the rating given to the Whale gallery and compares whether the result is greater than 0. For example, you could use this expression to select respondents for whom the difference between the two ratings is greater than zero.

This example uses the remember and interest categorical variables again. remember is a multiple response variable that records which galleries the respondents remember viewing, and interest is a single response variable that records which gallery respondents found most interesting. In this example, the - operator removes the responses given to the interest question from the responses given to the remember question. The >= operator then tests whether the resulting list of categories includes the Fossils category. You could use this expression to create a filter to select respondents who remember the Fossil gallery, but who did not find it the most interesting gallery.

The - operator is generally useful for categorical variables only if they have identical or overlapping category lists.