Levels of Measurement
There are four levels of measurement: Nominal, Ordinal, Interval, and Ratio. These go from lowest level to highest level. Data is classified according to the highest level which it fits. Each additional level adds something the previous level didn't have.
Nominal is the lowest level. Only names are meaningful here.
Ordinal adds an order to the names.
Interval adds meaningful differences
Ratio adds a zero so that ratios are meaningful.
