Today in Stats, we cover the next six pages of Chapter 7, on bivariate data and scatterplots. On these pages, the students learn how to make a scatterplot on their TI calculators. They also learn correlations, including the correlation coefficient r. Fortunately, there aren't that many differences between the TI-83 and TI-84 as far as scatterplots and their graphs are concerned. There are no TI-84 surprises of the type that we saw in the previous chapter on Normal distributions. I've heard of the correlation coefficient r before, but this is the first time I've ever seen the actual formula for calculating it: r = sigma/sum z _ x z _ y /( n - 1), where z _ x and z _ y are the standardized (that is, converted to z -scores) values of x and y . I like how the text explains it -- standardizing the z -scores centers the plot at the origin. Then if there is a positive correlation, then for most points z _ x and z _ y have the s...
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