[R] AIC and goodness of prediction - was: Re: goodness of "prediction" using a model (lm, glm, gam, brt,
Kingsford Jones
kingsfordjones at gmail.com
Fri Sep 11 19:29:48 CEST 2009
Hi Corrado,
Not being familiar with your research goals or data I can't make
recommendations, but I can suggest a couple of places to look for
information: Frank Harrell's Regression Modeling Strategies and his
Design library available on CRAN, and Hastie et al's The Elements of
Statistical Learning.
A couple more comments below..
On Thu, Sep 10, 2009 at 11:48 AM, Corrado <ct529 at york.ac.uk> wrote:
> Dear Kingsford,
>
> I apologise for breaking the thread, but I thought there were some more people
> who would be interested.
>
> What you propose is what I am using at the moment: the sum of the squares of
> the residuals, plus variance / stdev. I am not really satisfied. I have also
> tried using R2, and it works well .... but some people go a bit wild eyed when
> they see a negative R2 (which is perfectly reasonable when you use R2 as a
> measure of goodness of fit on prediction on a dataset different from the
> training set).
To get negative values I'm guessing you're using 1 - ((sum((obs -
pred)^2)) / (sum((obs - mean(obs))^2))? If so a negative value
indicates the model is a worse predictor than using a constant. Also
note the formula is just a linear transform of the one mentioned in my
last email.
>
> I was then wondering whether it would make sense to use AIC: the K in the
> formula will still be the number of parameters of the trained model, the "sum
> of square residuals" would be the (predicted - observed)^2, N would be the
> number of samples in the test dataset. I think it should work well.
>
Generally, when assessing predictive ability one is not concerned with
the number of parameters -- just how good the predictions are on data
that is independent of the model selection and fitting process. Also,
the general definition of AIC uses likelihoods not SS residuals.
Also, using the SS resids you are once again back to a linear
transormation of the MSE estimate...
Kingsford
> What do you / other R list members think?
>
> Regards
>
> On Thursday 03 September 2009 15:06:14 Kingsford Jones wrote:
>> There are many ways to measure prediction quality, and what you choose
>> depends on the data and your goals. A common measure for a
>> quantitative response is mean squared error (i.e. 1/n * sum((observed
>> - predicted)^2)) which incorporates bias and variance. Common terms
>> for what you are looking for are "test error" and "generalization
>> error".
>>
>>
>> hth,
>> Kingsford
>>
>> On Wed, Sep 2, 2009 at 11:56 PM, Corrado<ct529 at york.ac.uk> wrote:
>> > Dear R-friends,
>> >
>> > How do you test the goodness of prediction of a model, when you predict
>> > on a set of data DIFFERENT from the training set?
>> >
>> > I explain myself: you train your model M (e.g. glm,gam,regression tree,
>> > brt) on a set of data A with a response variable Y. You then predict the
>> > value of that same response variable Y on a different set of data B (e.g.
>> > predict.glm, predict.gam and so on). Dataset A and dataset B are
>> > different in the sense that they contain the same variable, for example
>> > temperature, measured in different sites, or on a different interval
>> > (e.g. B is a subinterval of A for interpolation, or a different interval
>> > for extrapolation). If you have the measured values for Y on the new
>> > interval, i.e. B, how do you measure how good is the prediction, that is
>> > how well model fits the Y on B (that is, how well does it predict)?
>> >
>> > In other words:
>> >
>> > Y~T,data=A for training
>> > Y~T,data=B for predicting
>> >
>> > I have devised a couple of method based around 1) standard deviation 2)
>> > R^2, but I am unhappy with them.
>> >
>> > Regards
>> > --
>> > Corrado Topi
>> >
>> > Global Climate Change & Biodiversity Indicators
>> > Area 18,Department of Biology
>> > University of York, York, YO10 5YW, UK
>> > Phone: + 44 (0) 1904 328645, E-mail: ct529 at york.ac.uk
>> >
>> > ______________________________________________
>> > R-help at r-project.org mailing list
>> > https://stat.ethz.ch/mailman/listinfo/r-help
>> > PLEASE do read the posting guide
>> > http://www.R-project.org/posting-guide.html and provide commented,
>> > minimal, self-contained, reproducible code.
>
>
>
> --
> Corrado Topi
>
> Global Climate Change & Biodiversity Indicators
> Area 18,Department of Biology
> University of York, York, YO10 5YW, UK
> Phone: + 44 (0) 1904 328645, E-mail: ct529 at york.ac.uk
>
>
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