A norm group is a reference group that is used to compare your respondent’s scores on a test or scale against similar others. This gives the score meaning. For example, a score of 20/30 (known as the raw score) means nothing on it’s own. We need to know how well similar others perform if it’s an ability test or whether a person is scoring in the middle band, above or below the middle band in a personality questionnaire. By comparing the score with a group of similar others, we add meaning and thus interpretation to our observed score.
NB: Not all psychometric tests have norm groups/norm tables. This is because some tests rely on comparison within the self for their interpretation. These tests are usually referred to as Ipsative or forced choice tests. Here we can say that a respondent is more sociable than questioning for example, but we cannot compare our respondent with others. It is due to this lack of comparison that ipsative tests should be reserved for development and coaching and not used in selection.