oops, then i guess i should not have sent the recode suggestion. choonhong: I only sent it as an example of how to recode your factor. I didn't mean to imply ( nor did i even give it much thought ) that what you're doing is statistically/philosophically
correct.

I'm a friend but I think what David is implying is that you are deciding on hypotheses after looking at results which is kind of cheating and means that you can't rely on any statistical tests that you do going forward because they will be biased.

I'm sure he has a good point but I don't want to get into this since I really don't know what you're doing and it's a very complex topic. I think Frank's book has a lot to say about this type of thing.






On Thu, Feb 26, 2009 at 12:14 AM, David Winsemius wrote:

You don't.

And even if you do get someone to tell you how, you may still not legitimately lower your degrees of freedom. Friends don't let friends use stepwise approaches to regression analysis.

--
David Winsemius

On Feb 25, 2009, at 10:33 PM, choonhong ang wrote:

The district a is the baseline and we observe the difference between
District a & b is not significant, we can choose to combine these 2 values.
How to write code to combine these 2 value ?

m1=glm(Claims~District+Group+Age+log(Holders),fami ly=poisson,data=mydata)
summary(m1)

Call:
glm(formula = Claims ~ District + Group + Age + log(Holders),
family = poisson, data = mydata)

Deviance Residuals:
Min 1Q Median 3Q Max
-2.553115 -0.471819 0.002411 0.455274 1.800739

Coefficients:
Estimate Std. Error z value Pr(>|z|)
(Intercept) -2.777752 0.689162 -4.031 5.56e-05 ***
Districtb 0.119942 0.079861 1.502 0.133125
Districtc 0.228371 0.144503 1.580 0.114019
Districtd 0.571661 0.248792 2.298 0.021576 *
Group>2l 0.794721 0.180354 4.406 1.05e-05 ***
Group1-1.5l -0.003496 0.127947 -0.027 0.978202
Group1.5-2l 0.379190 0.055856 6.789 1.13e-11 ***
Age>35 -1.074971 0.389480 -2.760 0.005780 **
Age25-29 -0.332131 0.129512 -2.564 0.010333 *
Age30-35 -0.539815 0.160138 -3.371 0.000749 ***
log(Holders) 1.201696 0.144135 8.337 < 2e-16 ***
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

(Dispersion parameter for poisson family taken to be 1)

Null deviance: 4236.68 on 63 degrees of freedom
Residual deviance: 49.45 on 53 degrees of freedom
AIC: 388.77
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