Minitab's 2 Proportions test uses a normal approximation by default to perform the hypothesis test and calculate the confidence interval. The normal approximation can be used to approximate the difference between two binomial proportions provided the sample sizes are large and proportions are not too close to 0% or 100%. In addition, when you specify a test difference of zero in the Options sub-dialog box, Minitab does Fisher's exact test, which is exact for all sample sizes and proportions. Minitab also provides an alternative normal approximation test based on the pooled estimate of the proportion.
The normal approximation may be inaccurate for small numbers of events or nonevents. If the number of events or nonevents in either sample is less than five, Minitab displays a note. Fisher's exact test is accurate for all sample sizes and proportions.
When the hypothesized difference between the proportions is 0 and the sample sizes are equal, the test based on separate estimates of the proportions is better than the one based on the pooled estimate of proportion.