If the sample sizes are larger, that is both n1 and n2 are greater than 30, then one uses the z-table.
If either sample size is less than 30, then the t-table is used.
- If n1 > 30 and n2 > 30, we can use the z-table:
Use Z table for standard normal distribution
- If n1 < 30 or n2 < 30, use the t-table:\
Use the t-table with degrees of freedom = n1+n2-2
For both large and small samples Sp is the pooled estimate of the common standard deviation (assuming that the variances in the populations are similar) computed as the weighted average of the standard deviations in the samples.
