= Zero-residual degeneracy of regression prediction
When a full-rank normal regression with positive residual degrees of freedom happens to fit all supplied observations exactly, its residual estimate of <variance> is zero. The ordinary Studentized prediction-interval formula formally collapses, but the statistic divides by zero. Under positive true Gaussian <variance> such samples form a null set; exact rounded or constructed data must not be treated as proof that future randomness vanishes.
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