The American psychologist Stanley Smith Stevens (1906–73) made a further sub-division with “continuous variables” in 1947 when he introduced ratio and interval scales of measurement (most of Pearson’s continuous variables were ratio). Stevens proposed the following: 1. Ratio scales These differ from interval variables (see here) in two ways: a) an absolute zero indicates the absence of the property being measured (i.e. height, weight and blood pressure) and b) ratio scales are additive.
