LECTURE 5
Frequency Distributions
SP110 STATISTICS FOR PSYCHOLOGY

OUTLINE
Frequency Distributions
Shapes of Frequency Distributions

Frequency Distributions
•
A frequency distribution shows the pattern of
frequencies over the various values.
•
A frequency table or histogram describes a frequency
distribution because each shows the pattern or shape of
how the frequencies are spread out, or “distributed.”
•
Describing the shape of a distribution is important, both
in the descriptive statistics and also in the inferential
statistics.

Shapes of Frequency Distributions
Unimodal, Bimodal, Multimodal and Rectangular Distributions
•
Unimodal:
Frequency distribution with one value clearly having a larger
frequency than any other. (One peak, one value has clearly a larger
frequency)
•
Bimodal:
distribution with two approximately equal frequencies, each
clearly larger than any of the others. (Two peaks, two approximately equal
frequencies)
•
Multimodal:
Frequency distribution with two or more high frequencies
separated by a lower frequency. (Two more high frequencies)
•
Rectangular:
Frequency distribution in which all values have approximately
the same frequency. (All values have about the same frequency)

•
These figures are not histograms, but special line graphs called frequency
polygons, which are another way to graph a frequency table.
•
In a
frequency polygon
, the line moves from point to point.
•
The height of each point shows the number of scores with that value. This
creates a mountain-peak skyline
Shapes of Frequency Distributions
Unimodal, Bimodal, Multimodal and Rectangular Distributions

•
Strictly speaking, a distribution is bimodal or
multimodal only if the peaks are exactly equal.
However, psychologists use these terms more
informally to describe the general shape.
•
The scores from most psychology studies are
usually an approximately unimodal distribution.
Bimodal and other multimodal distributions
occasionally turn up.
Shapes of Frequency Distributions
Unimodal, Bimodal, Multimodal and Rectangular Distributions