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The Behrens–Fisher t′ statistic can be employed when one seeks to make inferences about the means of two normal populations without assuming the variances are equal. The statistic was offered first by W. U. Behrens in 1929 and reformulated by Ronald A. Fisher in 1939:

where sample mean and sample variance s21 are obtained from the random sample of size n1 from the normal distribution with mean μ1 and variance has a t distribution with v1 = n1–1 degrees of freedom, the respective quantities with subscript 2 are defined similarly, and or . The distribution of t′ is the Behrens–Fisher distribution. It is, hence, a mixture of the two t distributions. The problem arising when one tries to test the normal population means ...

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