HOW MANY ARE BELOW YOU?

Your son has secured the fourth rank in the class. There are twenty students in the class. Hence 15 students are below him or 80% of students are below him. Now, your son is said to at the "80th percentile".

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

Formula:

Percentile = (Number of values below "x"/total number of values)*100

In our case

=(16/20)*100

=80

Suppose, your son has scored 90% marks in math. But has percentile in math as 20. That means, only 20% of the students are below him. That is not impressive in spite of the high score. So percentile gives new meaning to data.

A total of 10000 people visited the shopping mall for over 12 hours.

Time (hours) people

0 0

2 350

4 1100

6 2400

8 6500

10 8850

12 10000

1. Estimate the 30th percentile

when 30% of the visitors had arrived

2. Estimate what percentile of visitors had arrived after 11 hours.

These questions can be answered by drawing graphs easily. But doing calculations is a bit difficult.

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We draw a curve by taking hours in the x-axis and no. of people on the y-axis.

1. The 30th percentile occurs when the visits reach 3000 (since 30% of 10000 is 3000). Draw a horizontal line from 3000 until you hit the curve, then draw a line vertically downwards to read off the time on the x-axis.

the 30th percentile occurs at about 6.5 hours.

2. 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 vertical axis. So the visits at 11 hours were about 9500, which is the 95th percentile.

Uses:

Test takers can compare their performance to that of other students. Percentiles are used in children's growth charts (height, weight). This allows for an effective means of comparison so that parents can know if their child's growth is typical or unusual.

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