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In a talk on RED method, the presenter defends his use of 'average' by the following comparison with latencies. The linked-to part of the talk written down (non-verbatim and with my formatting):

There's 2 reasons why I plot averages:

  1. Latencies don't sum. If you got
  • A) 1 service that has high latency and you look at the 2 services it calls. Often it happens that the latter 2 have low latency. And this doesn't help you. Because latencies don't sum like that.
  • B) Averages (on the other hand) do sum. So if you got 1 service with high average, then one of them below, one of the 2 mentioned services it calls, is gonna have a high average. And that helps you pinpoint.
  1. (2nd reason omitted.)

What does he mean by, if I may summarise it, 'latencies don't sum, while averages do'. Also, wouldn't we be able to immediately pinpoint the callee service as the problematic one (as it's the only one with a high latency)?

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