[R] how to get correct coefficients from lm model

[Martin Henry H. Stevens]

On May 18, 2006, at 10:37 AM, Kum-Hoe Hwang wrote:

> Howdy
>
> I apologize for duplicated posting. But I decided to correct my  
> previous
> posting.
>
> I had the regression results using
>
> r <-  lm(Y ~ nemp + as.factor(devt), data=d).
>
Can you tell us how many levels of devt there are?
Do
levels(d$devt)

> First, there is the result of anova(r). Here I could not find  
> regression
> coefficients.
>
> Response: Y
>                        Df Sum Sq Mean Sq F value  Pr(>F)
> nemp                    1   58.2    58.2 1233.23 < 2e-16 ***
> as.factor(devt)        3    3.6     1.2   25.69 2.6e-16 ***
>
> Second, there is the result of summary(r) that I am confused
> with too many coefficients for a factor variable "devt".
> Because there are three coefficients for factor variables
> such as"as.factor(devt)A ", "as.factor(devt)B", "as.factor(devt)C".
>
> Coefficients:
>                                   Estimate Std. Error t value Pr(>|t|)
> (Intercept)                       as.factor(devt1)        3     
> 3.6     1.2
> 25.69 2.6e-16 ***   5.95e-02   10.19  < 2e-16 ***
Why is the as.factor(devt1) here? what is devt1?
> nemp                              2.87e-06   1.04e-07   27.63  <  
> 2e-16 ***
> as.factor(devt)A            2.44e-02   1.47e-02    1.66  0.09630 .
> as.factor(devt)B           -1.10e-01   1.40e-02   -7.90  4.6e-15 ***
> as.factor(devt)C          -9.19e-03   1.53e-02   -0.60  0.54953
>
> Is this model right  as a tentative model at least?
>    Y = 6.07e-01  + 2.87e-06* nemp + 2.44e-02 * as.factor(devt)A
>          - 1.10e-01 * as.factor(devt1)B - 9.19e-03 * as.factor(devt)C
>
>
>  And so, my question is "which coefficients should I use for a final
> model?".
>
> Thanks in advance,
> -- 
> Kum-Hoe Hwang, Ph.D.Phone : 82-31-250-3516Email : phdhwang at gmail.com
>
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>
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Dr. M. Hank H. Stevens, Assistant Professor
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Botany Department
Miami University
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