Mean
The "mean" is simply the everyday average you already know. It tells you the typical or central value of a group of numbers by blending them all together into one representative figure.
To calculate it, you add up every value in your set, then divide that total by how many values there are. In the formula, x₁, x₂, and so on are the individual values, and n is the count of how many values you have.
A project manager uses the mean to summarize data quickly — for example, the average cost of tasks, the average time to close a support ticket, or the average number of defects per release. One caution when reading it: the mean can be pulled up or down by a single extreme value, so a very high or very low outlier can make the average less "typical" than it appears.
Suppose five tasks took 4, 6, 5, 9, and 6 days to finish. Add them: 4 + 6 + 5 + 9 + 6 = 30. Count the values: there are 5 of them. Divide: 30 ÷ 5 = 6. So the mean task duration is 6 days — a handy single number to describe how long a typical task takes.
Every PMP formula explained free — plus worked examples and practice in PMP Math, and full timed mocks in the simulator.