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Date:         Wed, 25 Feb 2009 11:10:18 -0500
Reply-To:     Peter Flom <>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Peter Flom <peterflomconsulting@MINDSPRING.COM>
Subject:      Re: oversampling too much?
Comments: To: Gary <fuguoyi@GMAIL.COM>
Content-Type: text/plain; charset=UTF-8

Gary <fuguoyi@GMAIL.COM> wrote

>I am new to this group, and just started a job with a bank. When >modeling rare events in marketing, it has been suggested by many to >take a sample stratified by the dependent variable(s) in order to >allow the modeling technique a better chance of detecting a >difference. Many literature suggests the proportion of the event in >the sample seems to range between 15-50% for a binary outcome, and we >can use an offset to adjust it. > >The response rate of my current case is 0.3%, and when I build the >model, I oversmapled the response to 25%. However, the trandition here >is to oversample to 1%, and they told me that if oversample too much, >the model will be sensitive. > >Is there any problem oversample from 0.3% (8000 out of 2.2M targets) >to 25% (8000 resps and 24000 non-resps). We have about 500 variables >to build the model. >

I am not an expert on this area, but 1) I don't see how oversampling from an existing data set helps. I could see oversampling when *building* a data set. You want to oversample rare populations so that you have enough people from those populations. But in your situation, I think the only advantage of oversampling would be the speed with with the logistic regression runs.

(That's just my intuition ....)

2) I am concerned with any model that has 500 variables, *regardless* of the number of cases. The rule of thumb of 10-1 is not bad, but it's not ironclad. What are these 500 variables? How are they related?

3) Since you are in marketing, I imagine you are mainly or entirely interested in prediction, rather than explanation. You might consider multimodel averzging (see a book by Burnham and Anderson)



Peter L. Flom, PhD Statistical Consultant www DOT peterflomconsulting DOT com

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