The use of Z or t again depends on whether the sample sizes are large (nstep one > 30 and n2 > 30) or small. The standard error of the point estimate will incorporate the variability in the outcome of interest in each of the comparison groups. If we assume equal variances between groups, we can pool the information on variability (sample variances) to generate an estimate of the population variability. Therefore, the standard error (SE) of the difference in sample means is the pooled estimate of the common standard deviation (Sp) (assuming that the variances in the populations are similar) computed as the weighted average of the standard deviations in the samples, i.e.:
Brand new trust interval might be calculated using both the fresh Z otherwise t shipments into chose depend on level plus the simple error of one’s point guess
If the sample sizes are larger, that is both n1 and n2 are greater than 30, then one uses the z-table.