Q-Q Plot
A Q-Q plot (quantile-quantile plot) is a visualization technique for testing normality.
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Description
Rankits are expected quantiles of a 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 uniformly distributed variable would appear as an S-curve.
- Outliers appear far from the line.
It is particularly common to visualize a model's residuals using Q-Q plots, but this method can be applied to any supposedly normal variable.