Citation Hunt

The Wikipedia snippet below is not backed by a reliable source. Can you find one?

Click I got this! to go to Wikipedia and fix the snippet, or Next! to see another one. Good luck!

In page Weighted arithmetic mean:

This text has been unreferenced on Wikipedia for a very long time. If you can't find a source, be bold and remove it!

"

For uncorrelated observations with variances σ i 2 {\displaystyle \sigma _{i}^{2}} , the variance of the weighted sample mean is[citation needed]