  An exact method of calculating the variance of a pooled data set is
presented. Its major advantages over the many other methods are that it is
simple, is easily derived and remembered, and requires no assumptions. The
result can be concisely summarized as follows: "The Exact pooled variance is
the mean of the variances plus the variance of the means of the component data
sets." The proof is so simple that it has certainly been done many times
before, but it is absent in the textbooks. Its practical significance is
discussed.
