Variance

Variance measures how spread out your numbers are — in other words, how much they jump around instead of all sitting close together. Before you can find it, you need the mean, which is the average: you add up all the values and divide by how many there are. In the formula, "m" is that mean, each "xᵢ" is an individual value, and "n" is the number of values.

To calculate it in words: first find the mean. Then, for each value, subtract the mean and square the result (multiply it by itself). Squaring does two helpful things — it makes every difference positive, and it gives extra weight to values that are far from the mean. Finally, add up all those squared differences and divide by how many values you have. That average of the squared differences is the variance.

Reading the result is about big versus small: a large variance means the data is very scattered, with values far from the average, while a small variance means the values cluster tightly around the average. A project manager uses variance to judge how consistent or risky something is — for instance, whether task durations are predictable or all over the place. On its own the number is hard to interpret because it's in "squared" units, which is exactly why we usually take its square root to get the standard deviation.