# If we know stdev from the sample and they're different, how do we assume variances are equal?

If we calculate and know both variance and stdev of the sample, how do we assume equal variances? Is this an assumption for the population?

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And if we assume that it' variance for population, why don't we use z-score?

Hey Avtandil

we dont know the value of the POPULATION variances, but not THE SAMPLE variances.

We use T table when the sample size is small and the population variance is unknown.