Two processes with the same mean

Process A: every instance takes four minutes, without exception.

Process B: 90% take ninety seconds, 10% take twenty-eight minutes.

Both average four minutes. They are completely different problems. A is boring and easy to automate. B has a long tail that probably represents a different task entirely — the exceptions, the disputes, the ones where somebody has to go and ask.

In B, the 10% is over 70% of the total time. Automating the 90% saves less than a third of the effort and leaves the painful part untouched. That is a genuinely useful thing to know before you build, and the mean hides it completely.

Plot it, do not summarise it

A histogram of your baseline week. Not a mean, not a median — the actual shape.

Look for a second hump. Most processes that feel painful have one, and it is almost always a different underlying task that has been filed under the same name.

The 90th percentile

The time below which 90% of instances complete. Together with your median from the previous lesson this gives you the shape — and the ratio between them is the number worth watching.

Median 2 min, p90 24 min

Your baseline week gives a median of 2 minutes and a 90th percentile of 24 minutes. Total volume is 4,000 a month.

The shape of the work

You have just learned something about this process that the people performing it may not have articulated.