R help - Mar 2019 - ref.df in mgcv gam

Hello Group.

Regarding degrees of freedom in GAM models fit by mgcv, I do understand the
estimated df, but what does "ref.df" in the summary output mean?

I didn't see an explanation in Wood (2017) or find it in the function
code.  The answer may be on page 222 of

Wood SN. On p-values for smooth components of an extended generalized
additive model. Biometrika 2013;100(1):221-228

or thereafter, but I am not a statistician.  I could use help understanding
what the reference degrees of freedom are.

Thank you,

Susan




-- 
Susan Elias
Vector-borne Disease Lab, Maine Medical Center Research Institute

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These 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.

best,

Simon

On 27/03/2019 13:58, Susan Elias wrote:
> Hello Group.
>
> Regarding degrees of freedom in GAM models fit by mgcv, I do understand the
> estimated df, but what does "ref.df" in the summary output mean?
>
> I didn't see an explanation in Wood (2017) or find it in the function
> code.  The answer may be on page 222 of
>
> Wood SN. On p-values for smooth components of an extended generalized
> additive model. Biometrika 2013;100(1):221-228
>
> or thereafter, but I am not a statistician.  I could use help understanding
> what the reference degrees of freedom are.
>
> Thank you,
>
> Susan
>
>
>
>
-- 
Simon Wood, School of Mathematics, University of Bristol, BS8 1TW UK
https://people.maths.bris.ac.uk/~sw15190/

Thank you, Dr. Wood, that is very helpful.

Best,

Susan


On Wed, Mar 27, 2019 at 12:10 PM Simon Wood <simon.wood at bath.edu>
wrote:
> These 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.
>
> best,
>
> Simon
>
> On 27/03/2019 13:58, Susan Elias wrote:
> > Hello Group.
> >
> > Regarding degrees of freedom in GAM models fit by mgcv, I do
understand
> the
> > estimated df, but what does "ref.df" in the summary output
mean?
> >
> > I didn't see an explanation in Wood (2017) or find it in the
function
> > code.  The answer may be on page 222 of
> >
> > Wood SN. On p-values for smooth components of an extended generalized
> > additive model. Biometrika 2013;100(1):221-228
> >
> > or thereafter, but I am not a statistician.  I could use help
> understanding
> > what the reference degrees of freedom are.
> >
> > Thank you,
> >
> > Susan
> >
> >
> >
> >
> --
> Simon Wood, School of Mathematics, University of Bristol, BS8 1TW UK
> https://people.maths.bris.ac.uk/~sw15190/
>
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> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
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> PLEASE do read the posting guide
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> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Susan Elias
PhD Candidate, School of Earth and Climate Sciences, University of Maine
Research Associate, Vector-borne Disease Lab, Maine Medical Center Research
Institute
207-756-2761

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