An estimate obtained by combining information from two or more independent samples taken from populations believed to have the same mean. Observations x11, x12,…, x1m are randomly selected from a population. Their mean is x̄1, given byRandom observations from a second population are denoted by x21, x22,…, x2n and have mean x̄2. If the two populations are believed to have the same mean, then a pooled estimate of the common mean is x̄, given byWith k samples of sizes n1, n2,…, nk and with means x̄1, x̄2,… x̄k, the pooled estimate is given byThe unbiased estimate of the variance of the first population is , given byThe corresponding estimate for the second population is . If it is believed that the two populations have the same variance, but possibly different means, then the pooled estimate of common variance is s2, given byIn the case of k samples, with the estimate from sample j being , the pooled estimate is given bySee also hypothesis test.