R2 is most often seen as a number between 0 and 1, used to describe how well a regression line fits a set of data.

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He knows this because he has actual data on sales (available from Amazon) and the sales rank, and he knows how to plot a regression line in Excel.

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An R2 near 1 indicates that a regression line fits the data well, while an R2 close to 0 indicates a regression line does not fit the data very well.

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Using such intervals communicates the uncertainty within the model introduced by the scatter of the data instead of using the regression line that makes the relationship appear far more deterministic than it really is.

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