Suppose you are conducting a hypothesis test to compare two sample means from independent samples, with the variance unknown, but you know it is the same for both populations. Then you use the pooled estimate of the variance given by [ (n1 - 1)s1^2 + (n2-1)s2^2 ] / (n1+n2-2)(adsbygoogle = window.adsbygoogle || []).push({});

I was just wondering why we use (n1-1) etc instead of using n1 and n2 and then dividing by n1 + n2?

thanks

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# Pooled variance

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