Date:         Wed, 2 Jan 2008 14:56:39 -0600
Reply-To:     Mary <mlhoward@avalon.net>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Mary <mlhoward@AVALON.NET>
Subject:      Re: A conceptual question about regression
Comments: To: Tommy Xie <tommy.xie@GMAIL.COM>
Content-Type: text/plain; charset="iso-8859-1"

Tommy,

It doesn't really seem quite right to me.   For instance, I do opthalmology research and the presence of drusen (inflammatory spots) almost always occurs before the occurance of the disease AMD, and a certain GENE SNP, a factor H SNP, would occur at birth (since it is inherited or an abnormality in the gene occurs at birth.  So in this example would we want to have a model saying that we want to predict the factor H abnomality given independent variables of having AMD or not and having drusen or not?

In this example, it doesn't seem quite right to me- the dependent variable would seem to be the final event, not the initial event.

Thus a model

model amd_disease= b0 +  b1*factorh_snp + b2*drusen_present;

makes more sense to me than a model like

model factorh_snp = b0 + b1*drusen_present + b2*amd_disease;

So I'd agree with you theoritically, it seems backward to use the event that first occurs.   Perhaps you could let us know what your variables are to see if there might be a situation that makes more sense.

-Mary
  ----- Original Message ----- 
  From: Tommy Xie 
  To: SAS-L@LISTSERV.UGA.EDU 
  Sent: Wednesday, January 02, 2008 2:12 PM
  Subject: A conceptual question about regression


  Hi all,

  I'd appreciate if anyone can help me answer this question. If variable
  X takes place before Y and Z, which may indicates some sort of causal
  effect, is it O.K. to run a regression like X=B1*Y+B2*Z? I suspect
  that the regression has a wrong direction.

  Thanks in advance!