∑ j = 1 N ( X j − a ) 2 / N {\displaystyle {\sqrt {\sum _{j=1}^{N}(X_{j}-a)^{2}/N}}}
∑ j = 1 N ( X j − a ) 2 N {\displaystyle {\sqrt {\frac {\sum _{j=1}^{N}(X_{j}-a)^{2}}{N}}}}