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st: =?utf-8?Q?RE=3A_RE=3A_st=3A_=C2=B4=C3=B0=C2=B8=C2=B4=3A_st=3A_How_to_cho?==?utf-8?Q?ose_a_proper_model_if_the_depend?==?utf-8?Q?ent_variable_is_within_bounds=3F?=


From   "Maarten Buis" <M.Buis@fsw.vu.nl>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: =?utf-8?Q?RE=3A_RE=3A_st=3A_=C2=B4=C3=B0=C2=B8=C2=B4=3A_st=3A_How_to_cho?==?utf-8?Q?ose_a_proper_model_if_the_depend?==?utf-8?Q?ent_variable_is_within_bounds=3F?=
Date   Mon, 2 Oct 2006 11:59:23 +0200

Xiaoqiang Cheng:
You are fitting a nonlinear curve, so the effect of x changes as x changes. See for instance slide 21 in my presentation. With -mfx- you are summarizing this with one number, so you are approximating this non-linear curve with a straight line. In other words you are estimating the effect of a unit change for an average individual (default), or an individual with the values of x you specified. If the unit of your x is pretty small this approximation is usually fine, but if it is large you get strange results like you have just shown. -mfx- won't standardize the explanatory variable for you, so if you haven't standardized the variable uba, than -mfx- will show you the effect of a unit change, where the unit is the original unit in your dataset. The solution is to choose a smaller unit, for instance by dividing uba by some number (10 or 100) to improve the approximation.  
HTH,
Maarten

-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology 
Vrije Universiteit Amsterdam 
Boelelaan 1081 
1081 HV Amsterdam 
The Netherlands

visiting adress:
Buitenveldertselaan 3 (Metropolitan), room Z434 

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu]On Behalf Of Cheng, Xiaoqiang
Sent: maandag 2 oktober 2006 11:44
To: statalist@hsphsun2.harvard.edu
Subject: st: RE: st: ´ð¸´: st: How to choose a proper model if the dependent variable is within bounds?
However, there is a problem now:

-mfx- predicts that:

variable |      dy/dx    Std. Err.     z    P>|z|  [    95% C.I.   ]      X
uba|        -3.05144     1.55749   -1.96   0.050  6.10406  .001175   .004839

And the effect of uba on dependent variable exceeds unit if one std. dev. Of uba changes, certirus paribus?

 

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