R help - Jul 2006 - who can explain the difference between the R and SAS on the results of GLM

Dear friends,
 I used R and SAS to analyze my data through generalized linear model, and
there is some difference between them.



Results from R:

glm(formula = snail ~ grass + gheight + humidity + altitude + soiltemr +
airtemr, family = Gamma)



Deviance Residuals:

     Min        1Q    Median        3Q       Max

-1.23873  -0.41123  -0.08703   0.24339   1.21435



Coefficients:

                   Estimate Std. Error t value Pr(>|t|)

(Intercept)       2.024e-02  1.655e-02   1.223  0.22320

grasshuanghuacai  1.321e-02  5.053e-03   2.615  0.00982 **

grasshucao        1.962e-04  1.971e-03   0.100  0.92083

grassyuhao       -1.881e-03  2.041e-03  -0.922  0.35810

gheight          -1.275e-04  6.288e-05  -2.027  0.04441 *

humidity          6.797e-02  2.278e-02   2.983  0.00332 **

altitudelow      -5.090e-03  1.905e-03  -2.671  0.00837 **

soiltemr         -8.584e-04  5.165e-04  -1.662  0.09858 *.*   #is it show
that soiltemr maybe significant at a=0.05???

airtemr           6.547e-05  1.803e-04   0.363  0.71695

---

Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05
'.' 0.1 ' ' 1



(Dispersion parameter for Gamma family taken to be 0.2745989)



    Null deviance: 63.635  on 161  degrees of freedom

Residual deviance: 43.214  on 153  degrees of freedom

AIC: 1527.6



Results From SAS

*proc* *genmod* data=a order=data;

   class grass altitude;

   model snail = grass gheight humidity altitude soiltemr airtemr

              / dist=gamma type3;

*run*;





                                  Analysis Of Parameter Estimates



                                            Standard   Wald 95% Confidence
Chi-

  Parameter                 DF   Estimate      Error          Limits
Square   Pr > ChiSq



  Intercept                  1     0.0202     0.0160    -0.0111     0.0516
1.60       0.2052

  grass       hucao          1     0.0002     0.0019    -0.0035     0.0039
0.01       0.9179

  grass       yuhao          1    -0.0019     0.0020    -0.0057     0.0020
0.91       0.3397

  grass       huanghuacai    1     0.0132     0.0049     0.0037     0.0228
7.34       0.0068

  grass       diluo          0     0.0000     0.0000     0.0000
0.0000
.          .

  gheight                    1    -0.0001     0.0001    -0.0002    -0.0000
4.41       0.0358

  humidity                   1     0.0680     0.0220     0.0249     0.1111
9.55       0.0020

  altitude    low            1    -0.0051     0.0018    -0.0087    -0.0015
7.66       0.0057

  altitude    high           0     0.0000     0.0000     0.0000
0.0000
.          .

  soiltemr                   1    -0.0009     0.0005    -0.0018     0.0001
2.96       0.0852

  airtemr                    1     0.0001     0.0002    -0.0003     0.0004
0.14       0.7067

  Scale                      1     3.9077     0.4170     3.1702     4.8167



NOTE: The scale parameter was estimated by maximum likelihood.



   The GENMOD Procedure



                                 LR Statistics For Type 3 Analysis



                                                      Chi-

                            Source           DF     Square    Pr > ChiSq



                            grass             3      17.60        0.0005

                            gheight           1       4.26        0.0390

                            humidity          1       9.11        0.0025

                            altitude          1       7.67        0.0056

                            soiltemr          1       2.89        0.0889

                            airtemr           1       0.14        0.7050

Questions:

1.About the variable soiltemr: R could say it maybe significant at 0.05,
while SAS don't give this information,why was that in R?



2.Their dispersion parameters are different, although they are estimated
automatically,why?



3.From R's Results, i can write my model like this:

snail=1.321e-02* grasshuanghuacai+1.962e-04* grasshucao-1.881e-03*
grassyuhao-1.275e-04*gheight+6.797e-02*humidity-5.090e-03*altitudelow-8.584e-04*soiltemr

is it correct?

thanks very much!




-- 
Kind Regards,
Zhi Jie,Zhang ,PHD
Department of Epidemiology
School of Public Health
Fudan University
Tel:86-21-54237149

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