Table 1. The 4 levels of measurements.

The levels are compared using 4 measurement-related classes: resolution, property, mathematical operators, and central tendency [2].

Level	Measurement resolution	Measure property	Mathematical operators	Central tendency
Nominal	Lowest	Classification, membership	=, ≠	Mode
Ordinal	Low	Comparison, level	>, <	Median
Interval	High	Difference, affinity	+, -	Mean, deviation, variance
Ratio	Highest	Magnitude, amount	×, /	Geometric mean, coefficient of variation

Before proceeding to Rule 2, list all the relevant variables in your data set, and identify their types.