Ratios are used in many fields to adjust or normalize one measure for another in order to make comparisons or rankings. In economics, national indices for wealth are formed by the ratio of wealth to population size, examples being per capita income and gross national product per capita.
Regression analysis is the standard way to adjust one measure for another.
Any observations falling on the regression line are thought of as being equal relative to the covariate. Observations above the line are relatively large and ones below the line are relatively small. In regression analysis, a constant is included to estimate the value of the response variable when the covariate is at zero. A ratio is a special case of regression, equivalent to fitting without a constant or forcing the constant to be zero. In human biology, physiology, and nutrition, there is increasing awareness that the use of such ratios can lead to spurious results.