The covariance between two items p and q is computed using the equation:
 | (17–135) |
where:
| p = reference number of first item (input as IA on CVAR command) |
| q= reference number of second item (input as IB on CVAR command) |
| p and q can be displacements, stresses, or reaction forces. |
All other variables in Equation 17–135 are defined in Spectrum Analysis. When p = q, the above covariance becomes the variance.