= Q-Q Plot = A '''Q-Q plot''' ('''quantile-quantile plot''') is a visualization technique for testing [[Analysis/NormalDistribution|normality]]. <> ---- == Description == [[Statistics/Rankit|Rankits]] are expected quantiles of a [[Analysis/NormalDistribution|normal distribution]]. By plotting a set of observations according to a measurement (on the vertical axis) and the corresponding rankit (on the horizontal axis), deviations from normality are visualized and can be inspected for broader trends. * A straight line is indicative of a normally distributed random variable. * Curves suggest a skew. * A convex (opens upward) curve suggests a right skew. * A concave (opens downward) curve suggests a left skew. * An [[Analysis/UniformDistribution|uniformly distributed]] variable would appear as an S-curve. * Outliers appear far from the line. It is particularly common to visualize a model's [[Statistics/Residuals|residuals]] using Q-Q plots, but this method can be applied to any supposedly normal variable. ---- CategoryRicottone