# comparing populations

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• #25816

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I think this is an easy one. I would like to calculate a statisically significant sample size in comparing 2 designs.  I would like to assure that the new design produces fewer failures. Any suggestions.

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#62518

Robert Butler
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What you are describing is the calculation of a sample size to compare two proportions.  What you meant to ask is for a given difference in proportions what is the sample size needed to be sure that if the difference is significant you will be at least 80% certain that this significant difference is real
To answer this you will have to have an estimate of the proportions.  If the old design produces 20% failures and you think the new design will produce 15% failures then these become the proportions you will use for the sample size calculation.

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