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Re:Averages Can Be Misleading: Try a Percentile (2014) by LiamPa on Tuesday April 02, 2019 @03:30PM (#19557757)
Site Reliabilty Engineering goes over this in a lot more detail.

https://landing.google.com/sre/books/

Re:Averages Can Be Misleading: Try a Percentile (2014) by spenthil on Tuesday April 02, 2019 @06:40PM (#19559279)
Specifically Chapter 4, under "Aggregation" https://landing.google.com/sre/sre-book/chapters/service-lev...