Comparing a Proportional Sample with the Population (95% Confidence)

First, we must calculate a test value Z_t.

P = proportion of defects (or whatever) in the population (historical)

p = proportion of defects (or same as above) in the sample

|p – P|= absolute proportion difference (no minus sign in difference) n = sample size

If Z_t > 1.96, then we can say with a 95% confidence that the sample is different from the population.