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Hi I am struggling to understand Ref.df in the output screen in R:

Approximate significance of smooth terms:
               edf Ref.df     F p-value  
s(meangrain) 1.779  2.209 3.193  0.0451 *
s(depth)     2.108  2.697 3.538  0.0254 *

What does it mean and is it necessary to include this term for presenting results of GAM in a paper? Is it giving us information necessary for prediction?

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    $\begingroup$ Also would like to know this. I've searched the net but cant find an explicit answer. Is it possibly just the residual df under a different label? $\endgroup$
    – user93724
    Commented Oct 31, 2015 at 22:49

1 Answer 1

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Simon Wood, the author of the mgcv package and a book on GAMs, explains this here:

These [Ref.df] are a bit of a throwback really, and are not very useful - they are reference degrees of freedom used in computing test statistic and the p-values, but since the null distributions are non-standard the reference DoF is not very interpretable.

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