Step by step
The question
Question: draw a histogram of 2, 13, 25, 8, 17, 21, 34, 11, 19, 15, 28, 16, 23 and 12, in classes of 10.
How many are 20 or more?
1 Make the classes
The smallest number is 2 and the biggest is 34.
In classes of 10: 0 to 9, 10 to 19, 20 to 29 and 30 to 39.
2 Make a frequency table
A frequency table first: one row for each class.
2 is in 0 to 9: a mark there, and we cross out 2.
13 is in 10 to 19.
And so on for every number.
We count the marks: 2, 7, 4 and 1.
3 Draw the bars
Along the bottom: the numbers, from 0 to 40.
Up the side: how many.
0 to 9: 2 numbers.
The bar covers 0 to 10 and goes up to 2.
10 to 19: 7.
20 to 29: 4.
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: 4.
30 to 39: 1.
We add those bars: 4 plus 1 is 5.
5 Check
Let's check: all the bars together are 2 plus 7 plus 4 plus 1, which is 14.
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.