Arrange the following in terms of increasing order of the minimum number of variables required for their computation:

In increasing order of variables needed, the sequence is mean, correlation coefficient, first order correlation, second order correlation.

The mean needs only one variable, so it comes first.

A simple correlation coefficient needs two variables, placing it second.

A first order partial correlation controls for one variable and needs three, coming third.

A second order partial correlation controls for two variables and needs four, coming last.

The order of a partial correlation equals the number of variables held constant.