* R
    * pnorm, qnorm, rnorm, dnorm
    * pt, qt, rt, dt
    * <del> pf, qf, rf, df </del> 나중에
  * variance (var())
    * SS / df
  * standard deviation (sd{})
    * sqrt(SS/df)
    * 68, 95, 99 % 
  * [[:r:sampling distribution]] 아래와 거의 같은 내용
  * [[:r:hypothesis testing]]
  * [[:t-test summary]]