what is a z test 
a z test is a statistical test used to determine whether two population means are different when the variances are known and the sample size is large it can also be used to compare one mean to a hypothesized value the data must approximately fit a normal distribution otherwise the test doesn t work parameters such as variance and standard deviation should be calculated for a z test to be performed understanding z teststhe z test is also a hypothesis test in which the z statistic follows a normal distribution the z test is best used for greater than 30 samples because under the central limit theorem as the number of samples gets larger the samples are considered to be approximately normally distributed 