Welch–Satterthwaite equation

In statistics and uncertainty analysis, the Welch–Satterthwaite equation is used to calculate an approximation to the effective degrees of freedom of a linear combination of independent sample variances, also known as the pooled degrees of freedom, corresponding to the pooled variance. For n sample variances si2 (i = 1, ..., n), each respectively having νi degrees of freedom, often one computes the linear combination.

Source: Wikipedia — Welch–Satterthwaite equation (CC BY-SA 4.0)