How is the pooled SD calculated?
Quality Analyst 369A The equation for pooled SD (or variance = SD^2) is in the back of the Quality Analyst Manual under "Ordinary t-Statistic: Unpaired Samples". The sense of the equation is that you calculate the sum of squares for each sample independently, add them, divide by the pooled degrees of freedom, and take the square root to get SD. The calculation is similar to calculating a simple standard deviation. The only trick is you're calculating the sum of squared deviations from the mean for each group using each group's mean. The degrees of freedom is "m + n - 2" instead of "m + n - 1" because you're using two different means not one mean. QualityAnalyst 5.1 en
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