Histograms
Bar charts with bars touching; for unequal class widths use frequency density. Higher tier.
Higher tier. Bars touch (continuous data). Equal widths: height = frequency. Unequal widths: height = frequency density (frequency ÷ class width). Area of each bar = frequency.
Worked examples
Class 0-10 freq 30, width 10: density 3.
Class 10-25 freq 45, width 15: density 3.
Class 25-30 freq 25, width 5: density 5.
Frequently asked questions
Why density not frequency?
With unequal widths, frequency-only bars over-emphasise wide classes. Density makes each bar's AREA the frequency.
Estimating from a histogram?
Sum the bar areas in the range; for partial bars, multiply density by partial width.