Mean / Average

The mean (average) is the sum of all values divided by their count; the most common way to summarize a set of numbers in one central point.

The mean is simple and familiar, but that simplicity makes it dangerous because it is heavily pulled by outliers. In web data, whose distribution is often skewed — session duration, say, where most sessions are short and a few very long ones drag the tail — the mean is pulled toward that long tail and reads larger than the typical user's experience. Hence the rule: the mean is good when the distribution is roughly symmetric; on a skewed distribution, the median or a percentile gives a truer picture. The common mistake is reporting 'the average' without seeing the distribution behind it; two sets with the same mean can behave completely differently.