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Chapter 2

Ch. 2 - Frequency Distributions.docx

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York University
PSYC 2530
Anne Russon

Ch. 2 - Frequency Distributions Sunday, November 11, 2012 5:49 PM PSYC 2530 Introductory Statistics Chapter 2: Frequency Distributions  A frequency distribution can be a table or a graph, but in either case, the distribution presents the same two elements: o 1) The set of categories that make up the original measurement scale o 2) A record of the frequency, or number of individuals in each category  X = column heading for scores  f = column heading for frequencies  Σf = N  o Fraction of the total group that is associated with each score o Commonly a decimal   Grouped frequency distribution table: o About 10 class intervals o The width of each interval should be a relatively simple number (e.g., 2, 5, 10, 20) o The bottom score in each class interval should be a multiple of the width o All intervals should be the same width  Graphs: o X-axis = X values o Y-axis = frequencies  Two rules: o The point where the two axes intersect should have a value of zero for both the scores and the frequencies o The graph should be constructed so that its height (Y-axis) is approximately two-thirds to three-quarters of its length (X-axis)  Constructing a histogram: o The height of the bar corresponds to the frequency for that category o For continuous variables, the width of the bar extends to the real limits of the category. For discrete variables, each bar extends exactly half the distance to the adjacent category on each side o Adjacent bars touch and there are no spaces or gaps between bars  Modified histogram: o Draw a stack of blocks above each score, equal to the frequency o Eliminates the need for a Y-axis  Constructing a polygon: o A dot is centered above each score so that the vertical position of the dot corresponds to the frequency for the category o A continuous line is drawn from dot to dot to connect the series of dots o The graph is complete
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