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Unformatted text preview: 2.2 Correlation scatterplots display the relationship between two variables linear (straight-line) relationships are important because they are quite common linear relationship is strong if points lie close to a straight line linear relationship is weak if points are widely scattered about a line Figure 2.9 problem with scatterplot- our eyes can be fooled about the strength of the relationship need numerical measure of strength of linear relationship- correlation The correlation r measures the strength and direction of the linear relationship between two quantitative variables. Suppose that we have data on variables x and y for n individuals. The mean and standard deviation of the x-values are x and s x . The mean and standard deviation of the y-values are y and s y . The correlation r between x and y is: r = 1 1- n - ) ( x i s x x ) ( y i s y y- Suppose that x is height in centimeters and y is weight in kilograms and that we have height and weight measurements for n people. ) ( x i s x x- is the standardized height for the i th person ) ( y i s y y- is the standardized weight for the i th person ) ( x i s x x- = ) ( cm cm cm- = # with no units ) ( y i s y y- = ) ( kg kg kg- = # with no units Standardized values have no units of measurement. Standardized values have no units of measurement....
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- Spring '08