\documentclass{article} \usepackage{german} \begin{document} Each of the measurements $x_1 < x_2 < \cdots < x_r$ occures $p_1$-, $p_2$-$,\ldots,p_r$ times. The mean value and standard deviation are then \[ x = \frac{1}{n}\sum_{i=1}^r p_i x_i,\qquad s= \sqrt{\frac{1}{n}\sum_{i=1}^r p_i(x_i - x)^2} \] where $ n = p_1 + p_2 +\cdots+p_r $. \end{document}