The P_CORRELATE function computes the partial correlation coefficient of a dependent variable and one particular independent variable when the effects of all other variables involved are removed.

To compute the partial correlation, the following method is used:

- Let
*Y*and*X*be the variables of primary interest and let*C*..._{1}*C*be the variables held fixed._{p} - Calculate the residuals after regressing
*Y*on*C*..._{1}*C*. (These are the parts of_{p}*Y*that cannot be predicted by*C*..._{1}*C*.)_{p} - Calculate the residuals after regressing
*X*on*C*..._{1}*C*. (These are the parts of_{p}*X*that cannot be predicted by*C*..._{1}*C*.)_{p} - The partial correlation coefficient between
*Y*and*X*adjusted for*C*..._{1}*C*is the correlation between these two sets of residuals._{p}

This routine is written in the IDL language. Its source code can be found in the file p_correlate.pro in the lib subdirectory of the IDL distribution.

## Examples

; Define three sample populations:

X0 = [64, 71, 53, 67, 55, 58, 77, 57, 56, 51, 76, 68]

X1 = [57, 59, 49, 62, 51, 50, 55, 48, 52, 42, 61, 57]

X2 = [ 8, 10, 6, 11, 8, 7, 10, 9, 10, 6, 12, 9]

; Compute the partial correlation of X0 and X1 with the effects

; of X2 removed.

result = P_CORRELATE(X0, X1, X2)

PRINT, result

IDL prints:

0.533469

## Syntax

*Result* = P_CORRELATE( *X*,* Y*,* C* [, /DOUBLE] )

## Return Value

Returns the correlation coefficient.

## Arguments

### X

An *n*-element integer, single-, or double-precision floating-point vector that specifies the independent variable data.

### Y

An *n*-element integer, single-, or double-precision floating-point vector that specifies the dependent variable data.

### C

An integer, single-, or double-precision floating-point array that specifies the independent variable data whose effects are to be removed. *C* may either be an *n*‑element vector containing the independent variable, or a *p*-by-*n* two-dimensional array in which each column corresponds to a separate independent variable.

## Keywords

### DOUBLE

Set this keyword to force the computation to be done in double-precision arithmetic.

## Version History

4.0 |
Introduced |

## Resources and References

J. Neter, W. Wasserman, G.A. Whitmore, *Applied Statistics (Third Edition)*, Allyn and Bacon (ISBN 0-205-10328-6).

## See Also

A_CORRELATE, C_CORRELATE, CORRELATE, M_CORRELATE, R_CORRELATE