# Confidence intervals for mean, when variance is unknown

When calculating the confidence interval for the population mean and the variance is unknown, I take it that you have to use the t-distribution.

However, do you use the one-tailed or two-tailed test values for $\alpha/2$?

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This question doesn't make complete sense to me. You calculate confidence intervals for sample means. Yes, if the variance is unknown, you should use the t-distribution, rather than the normal. As for 1 vs 2 tailed CI's, you could use either, depending on what is more relevant for your substantive question. –  gung Nov 29 '12 at 5:00

$\bar{x}-t_{n-1,1-a}\frac{S}{\sqrt{n}}$
$\bar{x}+t_{n-1,1-a}\frac{S}{\sqrt{n}}$