APA Dictionary of Psychology

a type of statistical test that compares the means of two different samples to determine whether there is a significant difference between them (i.e., one not likely to have occurred by chance). Generally, this involves comparing the mean from a sample of a population to the mean for the whole population but may also involve comparing the means of two different populations. The z test is based on the normal distribution and is used when a population’s standard deviation is known or the sample is large (greater than 30). The equivalent t test is used with unknown standard deviations or smaller samples.