Percentiles do not average

Worked examples, recomputed

Every number the mechanisms quote for this relationship is recomputed from it, so none of them disagrees with it.

• "average p99" of 100 ms and 900 ms = 500 ms, which describes nothing

Where it appears

Each mechanism states it in its own names. The general symbol each name stands for is shown with it.

• Observability & Operations

  What is the smallest description of a running system from which we can find a fault, fast enough to matter, at a cost we can still pay while the system is failing?

  $$\text{average p99}\ne \text{fleet p99}$$

  Read it in place →

• Specialized Processing Models

  What do you build when a query predicate cannot be reduced to a range over an ordered key?

  $$\text{hourly p99}\ne f(\text{sixty minute-p99s})$$

  Read it in place →

• Measurement & Experimental Method

  What would have to be true for this number to mean what I think it means?

  $$\text{histograms add; percentiles do not}$$

  Read it in place →