Normal Distribution
nothing is called 95% until it has been counted
Concept

Lack of fit — where it appears

The part of a regression's residual variation that a different model could explain, as opposed to variation among repeated runs at the same setting. It can be tested only when some settings are replicated, since otherwise it cannot be told apart from pure error.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

A discrepancy of 4 σ at the top of the range: one bad run, or a line that bends there, in two designs of twelve runs. The same twelve errors in every panel. With one run at the top (upper panels) a bad observation and a bent line produce identical data, point for point. With three runs at the top (lower panels) the bad observation leaves one run far from its two neighbours and the bent line moves all three together. The dashed line is the true line; the grey line is the least-squares fit.

Three runs at the end of the line

With one run at the top of a regression's range, a bad observation there and a line that bends there produce data with exactly the same distribution, so no residual, influence measure or test can say which happened. Move six of twelve runs to the ends, three at each, and a discrepancy of four σ is named correctly as a bad run 84.1% of the time and as a bend 91.6% — while the slope's standard error falls from 0.0836 to 0.0709. The design, not the diagnostic, decides whether the question has an answer.

lineup · Qq What visiting fewer settings costs, 6 parameters. Carathéodory's bound puts the support of an optimal measure between 6 and 21. 6 settings: D-efficiency 88.90%, G-efficiency 57.18%, 0 degrees of freedom for lack of fit; 7 settings: D-efficiency 94.54%, G-efficiency 61.22%, 1 degrees of freedom for lack of fit; 8 settings: D-efficiency 95.99%, G-efficiency 64.60%, 2 degrees of freedom for lack of fit; 9 settings: D-efficiency 97.40%, G-efficiency 82.76%, 3 degrees of freedom for lack of fit. The saturated design has none, and buying the first one costs about five points of efficiency to get back.

How many places a design goes

Carathéodory's bound puts an optimal design's support between six and twenty-one settings, and every design in this field that can fit the model visits exactly nine. The count is not a choice anybody makes, it decides how many degrees of freedom are left to check the model with, and the first spare setting costs six points of efficiency to get back.

optimality · Equivalence

Named alongside it

The objects these essays reach for when they reach for this one.

Experimental designD-optimalityDegrees of freedomDesign measureG optimalityInfluenceInformation matrixLeverageModel diagnosticsOptimal designOutlierPrediction variance

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