Percentiles

Percentile: the value below which a percentage of data falls.

Example: You are the fourth tallest person in a group of 20

80% of people are shorter than you:

percentile 80%

That means you are at the 80th percentile.

If your height is 1.85m then "1.85m" is the 80th percentile height in that group.

In Order

Have the data in order, so you know which values are above and below.

To calculate percentiles of height: have the data in height order (sorted by height).
To calculate percentiles of age: have the data in age order.
And so on.

Grouped Data

When the data is grouped:

percentile rank 77%

Example: You Score a B!

In the test 12% got D, 50% got C, 30% got B and 8% got A

You got a B, so add up

for a total percentile of 12% + 50% + 15% = 77%

In other words you did "as well or better than 77% of the class"

(Why take half of B? Because you shouldn't imagine you got the "Best B", or the "Worst B", just an average B.)

Deciles

Deciles are similar to Percentiles (sounds like decimal and percentile together), as they split the data into 10% groups:

Example: (continued)

percentile 80%

You are at the 8th decile (the 80th percentile).

Quartiles

Another related idea is Quartiles, which splits the data into quarters:

Example: 1, 3, 3, 4, 5, 6, 6, 7, 8, 8

The numbers are in order. Cut the list into quarters:

Quartiles

In this case Quartile 2 is half way between 5 and 6:

Q2 = (5+6)/2 = 5.5

And the result is:

The Quartiles also divide the data into divisions of 25%, so:

Example: (continued)

For 1, 3, 3, 4, 5, 6, 6, 7, 8, 8:

Estimating Percentiles

We can estimate percentiles from a line graph.

shopping

Example: Shopping

A total of 10,000 people visited the shopping mall over 12 hours:

Time (hours) People
0 0
2 350
4 1100
6 2400
8 6500
10 8850
12 10,000

a) Estimate the 30th percentile (when 30% of the visitors had arrived).

b) Estimate what percentile of visitors had arrived after 11 hours.

First draw a line graph of the data: plot the points and join them with a smooth curve:

percentile graph

 

a) The 30th percentile occurs when the visits reach 3,000.

Draw a line horizontally across from 3,000 until you hit the curve, then draw a line vertically downwards to read off the time on the horizontal axis:

percentile graph 30% = 6.5

So the 30th percentile occurs after about 6.5 hours.

 

b) To estimate the percentile of visits after 11 hours: draw a line vertically up from 11 until you hit the curve, then draw a line horizontally across to read off the population on the horizontal axis:

percentile graph 11 = 95%

So the visits at 11 hours were about 9,500, which is the 95th percentile.