population variance = sigma^2 when the variance is unknown we use: sample estimator = s^2 but (xgem - u)/(s/rt(n)) does not follow the standard normal distribution. this distribution is called t distribution. (looks very much like the standard normal if n is large enough. ) so degrees of freedom are used. so xgem +/- t(n-1), 1-alfa/2 * (s/rt(n))

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