Step by step
The question
Question: draw a histogram of 12, 25, 31, 18, 27, 22, 35, 29, 15 and 24, in classes of 10.
How many are 20 or more?
1 Make the classes
The smallest number is 12 and the biggest is 35.
In classes of 10: 10 to 19, 20 to 29 and 30 to 39.
2 Make a frequency table
A frequency table first: one row for each class.
12 is in 10 to 19: a mark there, and we cross out 12.
25 is in 20 to 29.
And so on for every number.
We count the marks: 3, 5 and 2.
3 Draw the bars
Along the bottom: the numbers, from 10 to 40.
Up the side: how many.
10 to 19: 3 numbers.
The bar covers 10 to 20 and goes up to 3.
20 to 29: 5.
30 to 39: 2.
Careful!
The bars touch.
The numbers go on with no gaps, like a number line.
A bar chart has gaps; a histogram does not.
4 Read the histogram
20 to 29: 5.
30 to 39: 2.
We add those bars: 5 plus 2 is 7.
5 Check
Let's check: all the bars together are 3 plus 5 plus 2, which is 10.
That is every number in the list.
So it is right!
Let's look back.
First: make the classes.
Then: make a frequency table.
Then: draw the bars.
Then: read the histogram.
Last: check.