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Applied Supervised Learning with R
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Scale-Location plot shows whether residuals are spread equally along the ranges of input variables (predictor). The assumption of equal variance (homoscedasticity) could also be checked with this plot. If we see a horizontal line with randomly spread points, it means that the model is good.
The plot is between fitted values and the square root of standardized residuals. In the following figure, the top plot shows the linear function, and the residuals are spread randomly along the horizontal lines, whereas in the bottom plot, there seems to be a pattern that is not random. Hence, the variance is not equal:

Figure 4.6: [Top] Scale-Location plot of the linear function. [Bottom] Scale-Location plot of the quadratic function
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