Frequency is area, not height
On a histogram the bar's area equals the frequency. The height is the frequency density .
Two formulas, rearranged as needed
and .
Drawn from real examiner reports.
Reading the bar height as the frequency
Treating the vertical axis as frequency rather than frequency density.
Totalling bar heights instead of areas
Some students simply add the heights; you must use area = frequency.
Read the vertical scale carefully
Misreading the frequency-density axis is a frequent, avoidable error — check the gridline values.
Histograms are used when class intervals are unequal. The vertical axis is frequency density, not frequency: Rearranged: — this is the area of the bar.
What does the height of a histogram bar represent?
On a histogram, the bar for the class has a frequency density of . Find the frequency for this class.
