Answer for Do we use z-score or t-score when we have a large number of data with unknown population variance?

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Hi Harold,

Thanks for reaching out.

When we have unknown population variance we normally use the T-statistic.

However, after 100 degrees of freedom (when the sample is bigger than approximately 100), the T-table and the Z-table practically coincide. Therefore, it doesn’t really matter which one you use. 

The T distribution is developed to mimic the Normal distribution for small sample sizes. Therefore, it is no surprise that with more than 100 observations (when the sample becomes large enough) the two coincide (the T-distribution perfectly mimics the Normal distribution).

Best,

The 365 Team 

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