Step by step
The question
Question: draw a box plot of 10, 12, 15, 18, 20, 22 and 25.
A box plot shows how the numbers spread out.
The box holds the middle half.
1 Put them in order
First we put them in order, smallest first: 10, 12, 15, 18, 20, 22 and 25.
2 Find the median
7 numbers: 3 on each side of the middle one.
The median is 18.
3 Find Q1 and Q3
The median cuts the list in two halves, 3 numbers each.
The median itself is in neither half.
Careful!
The median is not counted in the halves.
Each half has 3 numbers.
Q1 is the middle of the lower half: 12.
Q3 is the middle of the upper half: 22.
4 Five-number summary
The five-number summary: the smallest 10, Q1 12, the median 18, Q3 22, and the biggest 25.
5 Draw the box plot
A number line from 10 to 26, counting in 2s.
The box goes from Q1, 12, to Q3, 22.
A line across the box at the median, 18.
The whiskers reach out to the smallest, 10, and the biggest, 25.
6 Check
Let's check: the median has 3 numbers below it and 3 above it.
Each half of the plot holds the same count.
So it is right!
Let's look back.
First: put them in order.
Then: find the median.
Then: find Q1 and Q3.
Then: five-number summary.
Then: draw the box plot.
Last: check.