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Comparing Proportions

The test for comparing proportions is used to test whether differences in the proportions of a characteristic that occur in two samples are sufficient to prove that the populations from which the samples were taken are different. It is used when the samples are large.

The hypothesis is:

H0: the proportion is the same in both populations
H1: the proportions are different/the proportion is greater for process 1 than process 2

The value of the Z statistic is calculated from: where:

p1 and p2: the proportions in each sample
X1 and X2: the number possessing the characteristic in each sample
n1 and n2: the sample size

The p-value can be obtained from Excel using the function:

one-tail test: = 1 - NORMSDIST(Z0)
two-tail test: = (1 - NORMSDIST(Z0))/2

Alternatively the critical values can be found in the t-tables. Analytical and descriptive statistics are covered in the MiC Quality online course Primer in Statistics. Try out our courses by taking the first module of the Primer in Statistics free of charge. [SIX SIGMA GLOSSARY ALPHABETICAL INDEX] [SIX SIGMA GLOSSARY INDEX OF TOPICS] [Top]

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