Histograms - Higher tier
A histogram looks like a bar chart, except the area of the bar, and not the height, shows the frequency of the data. Histograms are typically used when the data is in groups of unequal width.
Look at this table showing the various heights of different plants:
The frequency diagram below represents the data from the table with frequency being plotted on the y-axis.
Let’s look at a table with unequal class-widths:
When there are unequal class widths it is common to plot ‘frequency density’ rather than frequency. To calculate frequency density you divide the frequency for a group by the width of it.
For example, look at the first group, 5 \(\textless\) × \(\leq\) 11. This group has a frequency of six. We divide this by the width of the group which is also six, so the frequency density is one.
Looking at the second group, 11 \(\textless\) × \(\leq\) 16. This group has a frequency of 15 and a width of five. The frequency density is 15 ÷ 5 = 3.
The final group 16 \(\textless\) × \(\leq\) 18 has a class width of two and a frequency of four. The frequency density is 4 ÷ 2 = 2.
