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Z-Test vs T-Test
Based on the Wikipedia definitions for a Z-Test and T-Test:
*Difference between Z-test and t-test: Z-test is used when sample size is large (n>50), or the population variance is known. t-test is used when sample size is small (n<50) and population variance is unknown. … For large sample sizes, the t-test procedure gives almost identical p-values as the Z-test procedure.*
I see that most website A/B calculators and blogs advocate the use of T-Tests. Why wouldn’t a Z-Test be used at all times for digital advertising given the large sample sizes (thousands of impressions/clicks/conversions)? Also isn’t the population all users who were shown ads whether or not they clicked or converted, so you can calculate the variance?