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rough-notes-for-explaining-percentile
1.
- if 10 students took a test and scored the following:
10 20 30 40 50 60 70 80 90 100
and the cutoff percentage for a college admission is 90% - how many students got in?
- average does not help - avg = (n * (n + 1)) / 2 = 55
- throw out the bottom 90% of the scores
90 100
2 students scored 90 and above or 90% of the students scored 90 or below.
- What if the above was latency?
p90 is 90 - which says 90% of the requests took 90 or below
2. Better latency example
- 10 requests took the following time:
- 1 2 3 4 5 6 100 200 1 2 500
- sort
1 1 2 2 3 4 5 6 100 200 500
- total requests = 10
- How many requests took 5 ms or less?
- look and say - 7
- How many percent of requests experienced 5 ms or less? - Given ms - asking percentage
- sort
1 1 2 2 3 4 5 6 100 200 500
- find how many at or below 5 = 7
- how much percent is that of the total
100 * 7 / 10 = 70%
- 70% of requests experienced 5 ms or less
- What was the latency of 90% of my requests? - Given percentage - asking ms
- For a set of 10 requests
- 90% = 9
- Take the 9th value and discard the rest - 200ms
- p90 was 200ms i.e. 90% of requests experienced 200ms or less.
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