The Variance( obj ) command returns the variance of the quantity-with-error obj.

The quantity-with-error obj can have functional dependence on other quantities-with-error.

If the quantity-with-error obj does not have functional dependence on other quantities-with-error, the uncertainty of obj is accessed and converted to the variance (by squaring).

If the quantity-with-error obj has functional dependence on other quantities-with-error, the variance is calculated using the usual formula of error analysis involving a first-order expansion with the dependent form and covariances between the other quantities-with-error. This process can be recursive.

The variance  in , where  depends on the , is

where  is the error in ,  is the covariance between  and , and the partials are evaluated at the central values of the .

Variances involving physical constants are calculated naturally and correctly in the implied system of units because central values and errors are obtained from the interface to ScientificConstants.