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The Complete Idiot's Guide to Statistics, 2nd Edition by Robert Donnelly

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Correlation

Correlation measures both the strength and direction of the relationship between x and y. Figure 20.1 illustrates the different types of correlation in a series of scatter plots, which graphs each ordered pair of (x,y). The convention is to place the x variable on the horizontal axis and the y variable on the vertical axis.
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Figure 20.1
Different types of correlation.
Graph A in Figure 20.1 shows an example of positive linear correlation where, as x increases, y also tends to increase in a linear (straight line) fashion. Graph B shows a negative linear correlation where, as x increases, y tends to decrease linearly. Graph C ...

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