0580

Histograms

Statistics

Flashcards

Area, not height

  • A histogram displays grouped continuous data, usually in class intervals of unequal width
  • It looks like a bar chart but behaves differently in two ways that decide every question
    • The bars touch, with no gaps, because the data is continuous
    • The area of a bar gives the frequency, not its height
  • The vertical axis is labelled frequency density, which the syllabus states directly
  • Because area carries the meaning, a histogram cannot be read at a glance the way a bar chart can: some calculation is always needed
Bar chartHistogram
Datadiscrete or categoricalcontinuous, grouped
Class widthsequalusually unequal
Barsseparated by gapstouching
Vertical axisfrequencyfrequency density
Frequency shown bythe heightthe area
  • Where every class happens to be the same width, the vertical axis may simply say "frequency", and then the heights can be read directly
A histogram of weights with frequency density up the side and unequal class widths along the bottom: a wide low bar from 4 to 8 kg, a narrow tall bar from 10 to 12 with a density of 9.5, a middling bar from 12 to 15, and a very wide low bar stretching from 15 to 30
Source: Interpreting histograms by Save My Exams