\begin{thebibliography}{1}

\bibitem{var-incr-knuth}
Donald~E. Knuth.
\newblock {\em Seminumerical Algorithms}, volume~2 of {\em The Art of Computer
  Programming}, chapter 4.2.2, page 232.
\newblock Addison-Wesley, Boston, third edition, 1998.

\bibitem{mathworld-var-incr}
Eric~W. Weisstein.
\newblock Sample variance computation.
\newblock From Mathworld, a Wolfram web resource, \\
  \url{http://mathworld.wolfram.com/SampleVarianceComputation.html}.

\end{thebibliography}