CEVAP:

z test statistic can be used for difference between two means if sample sizes are greater than or equal to 30. In many real-life circumstances, it’s not possible or practical to attain the sample size of 30 or more due to the time, research budget and other relevant constraints. Also, usually the population standard deviations are unknown. Under these circumstances, t test is used to test the difference between means. At this time also, the assumption regarding the population distribution of the variables should be normal or approximately normal, and two samples are independently selected from these populations.