Following a Precision Study in Which Repeatability and Reproducibility of 20

Following a precision study in which repeatability and reproducibility of 20 samples are assessed,which one of the following formulae would be used to determine the total standard deviation ( $\sigma$ ²_T) ?

A) . $\sigma$ ²_within-run/2 + $\sigma$ ²_between-run
B) (x2_i - x1_i)
C) . $\Sigma$ (x₁ - $\mu$ ) ²/N
D) . $\Sigma$ ²_within-run + $\Sigma$ ²_between-run