Re: [R] can I do this with R?

From: Frank E Harrell Jr <f.harrell_at_vanderbilt.edu>
Date: Wed, 28 May 2008 17:32:28 -0500

Xiaohui Chen wrote:
> step or stepAIC functions do the job. You can opt to use BIC by changing
> the mulplication of penalty.
>
> I think AIC and BIC are not only limited to compare two pre-defined
> models, they can be used as model search criteria. You could enumerate
> the information criteria for all possible models if the size of full
> model is relatively small. But this is not generally scaled to practical
> high-dimensional applications. Hence, it is often only possible to find
> a 'best' model of a local optimum, e.g. measured by AIC/BIC.

Sure you can use them that way, and they may perform better than other measures, but the resulting model will be highly biased (regression coefficients biased away from zero). AIC and BIC were not designed to be used in this fashion originally. Optimizing AIC or BIC will not produce well-calibrated models as does penalizing a large model.

>
> On the other way around, I wouldn't like to say the over-penalization of
> BIC. Instead, I think AIC is usually underpenalizing larger models in
> terms of the positive probability of incoperating irrevalent variables
> in linear models.

If you put some constraints on the process (e.g., if using AIC to find the optimum penalty in penalized maximum likelihood estimation), AIC works very well and BIC results if far too much shrinkage (underfitting). If using a dangerous process such as stepwise variable selection, the more conservative BIC may be better in some sense, worse in others. The main problem with stepwise variable selection is the use of significance levels for entry below 1.0 and especially below 0.1.

Frank

>
> X
>
> Frank E Harrell Jr 写道:

>> Smita Pakhale wrote:
>>> Hi Maria,
>>>
>>> But why do you want to use forwards or backwards
>>> methods? These all are 'backward' methods of modeling.
>>> Try using AIC or BIC. BIC is much better than AIC.
>>> And, you do not have to believe me or any one else on
>>> this. 
>>
>> How does that help? BIC gives too much penalization in certain 
>> contexts; both AIC and BIC were designed to compare two pre-specified 
>> models. They were not designed to fix problems of stepwise variable 
>> selection.
>>
>> Frank
>>
>>>
>>> Just make a small data set with a few variables with
>>> known relationship amongst them. With this simulated
>>> data set, use all your modeling methods: backwards,
>>> forwards, AIC, BIC etc and then see which one gives
>>> you a answer closest to the truth. The beauty of using
>>> a simulated dataset is that, you 'know' the truth, as
>>> you are the 'creater' of it!
>>>
>>> smita
>>>
>>> --- Charilaos Skiadas <cskiadas_at_gmail.com> wrote:
>>>
>>>> A google search for "logistic regression with
>>>> stepwise forward in r" returns the following post:
>>>>
>>>>
>>> https://stat.ethz.ch/pipermail/r-help/2003-December/043645.html
>>>> Haris Skiadas
>>>> Department of Mathematics and Computer Science
>>>> Hanover College
>>>>
>>>> On May 28, 2008, at 7:01 AM, Maria wrote:
>>>>
>>>>> Hello,
>>>>> I am just about to install R and was wondering
>>>> about a few things.
>>>>> I have only worked in Matlab because I wanted to
>>>> do a logistic
>>>>> regression. However Matlab does not do logistic
>>>> regression with
>>>>> stepwiseforward method. Therefore I thought about
>>>> testing R. So my
>>>>> question is
>>>>> can I do logistic regression with stepwise forward
>>>> in R?
>>>>> Thanks /M
>>>> ______________________________________________
>>>
>>

>
>
-- 
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University

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