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st: fracpoly model


From   Chris Witte <eljefespeaks@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: fracpoly model
Date   Thu, 12 Mar 2009 13:20:17 -0700 (PDT)

I am using stata 9.2.  Here are the coefficient estimates from my _fracpoly_ analysis:

study1twsg       Coef.   Std. Err.      t    P>t     [95% Conf. Interval]
Itemp__1    15093.12   4454.121     3.39   0.012     4560.802    25625.45
Itemp__2   -10666.18    3136.84    -3.40   0.011    -18083.63   -3248.734
Itemp__3    1945.114   569.9302     3.41   0.011     597.4436    3292.785
_cons    .0053322   .0008244     6.47   0.000     .0033828    .0072815
Deviance:  -113.24. Best powers of temp among 164 models fit: -2 -2 -2.
Fractional polynomial model comparisons:
temp             df       Deviance      Res. SD      Gain   P(term) Powers
Not in model      0       -101.249      .002545        --     --
Linear            1       -101.599      .002641     0.000    0.603  1
m = 1             2       -101.796      .002617     0.197    0.714  3
m = 2             4       -104.278       .00248     2.679    0.508  3 3
m = 3             6       -113.237      .001764    11.638    0.196  -2 -2 -2

so, given these coefficients and my 'x' having a value of 25, I think my linear regression equation for predicting y should be:

y=.0053322+(15093.12*25^-2)+(-10666.18*25^-2*log(25))+1945.114*25^-2*(log(25))^2
 
if this is correct, the equation is not yielding the same estimates as _fracpred_.  The above equation returns an answer of 1.467, while _fracpred_ (which I believe is correct) gives 0.003.  
 
What am I doing wrong?
 
thanks a bunch in advance,
Chris



      

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