When the population variance or standard deviation is not known,you must use a t statistic instead of a z-score for a hypothesis test.

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When the population variance or standard deviation is unknown, you must estimate it using the sample data. This introduces additional uncertainty into the calculation, and the uncertainty is reflected in the t distribution rather than the standard normal distribution. Therefore, you must use a t statistic instead of a z-score for a hypothesis test when the population variance or standard deviation is not known.

When the Population Variance or Standard Deviation Is Not Known,you