I recently prepared a web report summarizing a few visitor trends when I realized (in spite of my liberal arts education) that a straight up average of all the stats was not going to do the job. Some referring sites were sending a ton of visits my way with the bottom-feeders sending over only a few dozen.

How could I weight those stats for the sites who were driving the most traffic? The answer: A weighted average (or weighted mean).

More practically, how could I accomplish this computation in Excel? A quick Google search revealed the following formula for Excel:
= SUMPRODUCT(B2:B15, C2:C15) / SUM(B2:B15)

I was looking for trends in pages/visit, % of new visitors and % bounce rate, among many other metrics.

Example (using fake web stats):

So, to find the weighted average for pages/visit, you would compute:
(400×5.3) + (35×2.1) + (230×1.5) …etc. / (400+35+230 …etc.)

Related: Simpson’s Paradox