Exercise 1
Compute RT interference scores for the Stroop task.
Exercise 2
If your version of SPSS is like mine, one of the levels of measurement will be incorrect. Can you work out which one it is and why it is incorrect?
Levels of measurement are defined by the type of operations you can meaningfully perform on the data. For example, scale-level data should allow you to compute means, ordinal-level data to compute medians and categorical-level data to compute frequencies.
Is there a scale variable for which it would not make sense to compute a mean?
It’s the participant ID. This is certainly numeric (see Type), but it is not at scale level (or ordinal level, for that matter). It is a categorical variable. The reason is that assignment of IDs to participants is completely arbitrary. It does not indicate a more or less of something we measured. Participant 1 is not “better” than, say, Participant 89.