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