Z-Test vs T-Test

Forum White Hat SEO PPC Z-Test vs T-Test

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  • #8809

    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?


    Are you talking about split testing ad copy and knowing when the difference between the values are high enough to conclude a result along with the confidence level?


    The real difference between Z-Test (Gauss Test) and T-Test is not the population size but the assumption about the standard deviation.

    In the Z-Test, you assume that you know exactly what the real standard deviation is (which is very unrealistically of course). In the T-Test you assume that you don’t know it so you have to estimate it first which is a more realistic scenario. This is the reason why the T-Test is used in the real world and not the Z-Test.


    It has been a while since I’ve taken my probability and statistics class so take this with a grain of salt.

    You are correct, T tests are used for a small n while z tests are used on a large n. BUT, t tests require you to use the sample variance while z tests require you to use the population variance. So they require different numbers. But I also remember our professor telling us that if we dont have a a table and just a t table to use the t table with infinite degrees of freedom to get our z value. So they are kind of interchangeable. But there is a difference and follow that rule to get more accurate results. But also 50 isnt the definined line between z and t tests. Some people consider 40 a big n and will use that as their boundary when to switch between z and t.

    Just remember it’s been a while since I’ve learned this so you might want to fact check this. I only posted because I didnt see anyone else and wanted to help a little.

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