rough-notes-for-explaining-percentile

understand-percentile-final.txt

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.