Unformatted text preview: 1 – μ 2 It clusters around a mean of 0, as predicted by H We calculate the standard error of the difference between means Calculating the standard error o There are 2 different formulas to use, depending on whether σ 1 2 (population variance for group 1) = σ 2 2 (population variance for group 2) o We will use only one – assume that σ 1 2 ≠ σ 2 2 Unequal standard deviations o For unequal population variances: σ 1  2 = ( (s 1 2 / n 11) + (s 2 2 / n 21)) o The formula for t is: t obt =( 1 2 )/ ( (s 1 2 / n 11) + (s 2 2 / n 21)) Degrees of freedom o For unequal standard deviations, it can be more complicated o DF=n 1 +n 22 is still a good estimate (contrary to what your book says)...
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 Spring '09
 Kupchick
 Statistics, Standard Deviation, Variance, Ronald Fisher

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