Could you please help me interpret the following residual plot and P-P plot from a multiple regression analysis?
I'd say that this shows evidence of heteroscedasticity as the residuals are grouped together, but I'm not sure.
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Heteroskedasticity is not about errors being grouped together but about unequal variance (variability) of the errors. In your plot errors seem to have different variability at the beginning of the plot then in the end so I would say there is heteroskedasticity there.
Probability-probability (p-p) plot measures how closely two distributions match together. If you get perfect straight lines the distributions are perfect match. However, p-p plots are used to test normality of errors by comparing the error distribution to normal one not to test heteroskedasticity.