4. How Variables Move Jointly: Correlation

Chapter 2, “How Values Cluster Together,” discussed how the values on one variable can tend to cluster together at an average of some sort—a mean, a median, or a mode. Chapter 3, “Variability: How Values Disperse,” discussed how the values of one variable fail to cluster together: how they disperse around a mean, as measured by the standard deviation and its close relative, the variance.

This chapter begins a look at how two or more variables covary: that is, how higher values on one variable are associated with higher values on another, and how lower ...

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