Formula
Frequency density
Statistics • Frequency Distribution • Frequency Density
Formula
The frequency density of a class is ; plotting it as the height of the bar makes the AREA of each bar equal to that class's frequency, which is what allows classes of different widths to appear in one histogram.
How to use it
- 1Take the width from the class boundaries, not from the stated limits, whenever the classes are inclusive.
- 2Use frequency density as the bar height whenever the class widths are unequal; with equal widths the plain frequency gives the same shape and is what is usually drawn.
- 3Read it backwards to recover a frequency from a histogram: frequency frequency density class width.
Worked example
A class has frequency . Find its frequency density.
Step-by-step
- 1
- 2
Why: Frequency divided by width.
Answer
Watch out
Dividing by a class limit instead of by the width is the standard error, and it is what the distractors in this kind of question are built from. A frequency density is a frequency PER unit of the variable, so it is not a count and need not be a whole number.
Builds on
Also known as
frequency density, histogram bar height, unequal class widths, frequency per unit width