Step by step
The question
Question: draw a box plot of 2, 4, 4, 5, 7, 8, 8, 9, 12 and 15.
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: 2, 4, 4, 5, 7, 8, 8, 9, 12 and 15.
2 Find the median
10 numbers: two in the middle, 7 and 8.
The median goes between them.
It cuts the list in two halves, 5 numbers each.
The median is halfway between them: we add them and divide by 2.
7 plus 8 is 15.
Divided by 2 is 7.5.
3 Find Q1 and Q3
Q1 is the middle of the lower half: 4.
Q3 is the middle of the upper half: 9.
4 Five-number summary
The five-number summary: the smallest 2, Q1 4, the median 7.5, Q3 9, and the biggest 15.
5 Draw the box plot
A number line from 2 to 16, counting in 2s.
The box goes from Q1, 4, to Q3, 9.
A line across the box at the median, 7.5.
The whiskers reach out to the smallest, 2, and the biggest, 15.
6 Check
Let's check: the median has 5 numbers below it and 5 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.