One main business for psychologists is to examine questionnaire data. Extraversion, intelligence, attitudes… That’s bread-and-butter job for (research) psychologists. Similarly, it is common to take the metric level of questionnaire data for granted. Well, not for the item level, it is said. But for the aggregated level, oh yes, that’s OK. Despite its popularity, the measurement basics of such practice are less clear. On which grounds can this comfortable practice be …