# Z-score: siga or s?

Why in the Z-test, and in the denominator you've used s (sample variance) instead of sigma (population variance)?

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In a Z-test, we use the sample standard deviation (s) in the denominator of the test statistic instead of the population standard deviation (σ) because we typically do not know the value of the population standard deviation. We use the sample standard deviation as an estimate of the population standard deviation.