Histogram
A histogram is a bar chart for continuous data grouped into class intervals, where bar area, not just height, represents frequency, making it the first tool for viewing a distribution's shape.
See it move
Fifty delivery times split into classes of 0–10, 10–20, 20–30 and 30–50 minutes, holding 5, 20, 15 and 10 orders. Because the last class is twice as wide, its raw frequency would look artificially short. Dividing each frequency by its class width gives densities of 0.5, 2.0, 1.5 and 0.5, so bar area, not height alone, reproduces the true frequency.
The formula
Variables
- Number of observations falling in the class interval (count)
- Width of the class interval on the horizontal axis
The correct bar height when class widths are unequal, so that bar area (not height alone) represents the class frequency.
Check yourself
A survey of 80 customers records waiting times grouped as: 0–5 minutes (10 customers), 5–10 minutes (30 customers), 10–20 minutes (24 customers), and 20–40 minutes (16 customers). When drawing a histogram, what should be the bar height for the 20–40 minute class?