Levels of measurement
Levels of measurement is the nominal-ordinal-interval-ratio hierarchy that classifies what a variable's numbers mean, which decides which averages, charts and tests are legitimate to use on it.
See it move
A retailer's store region is nominal — pure labels, no order. Store size (Small/Medium/Large) is ordinal — ordered, but gaps are unequal. A thermostat reading of 18°C is interval — equal gaps, but no true zero. Monthly revenue of €32,000 is ratio — equal gaps and a true zero, so a top store earning 1.29 times the lowest is valid.
Check yourself
A market research firm records four things for each of 300 survey respondents: (1) postcode area, (2) a 1-5 agreement scale on a statement, (3) age in years, (4) the room temperature in Celsius during the interview. Which of these four variables is measured at the ratio level?