Nonparametric statistics are designed to handle ordinal data. Interval scales consist of measures for which there are equal intervals between each discrete measures. You can assess the exact difference between two scores. Can be subtracted an otherwise manipulated. A population density of 20 is two times greater than a density of 10.
Elevation is the independent variable, and temperature is the dependent.
Rainfall and runoff. Rainfall is the independent variable, and runoff is the dependent. Birth rate and percent urban. Percent urban is the independent variable, and birth rate is the dependent.