[R] comparing matched proportions using glm
Charles C. Berry
cberry at tajo.ucsd.edu
Thu Oct 4 22:47:15 CEST 2007
On Thu, 4 Oct 2007, Corry Gellatly wrote:
>
> Dear R users,
>
> Is it possible to use a generalized linear model to do a binomial
> comparison of one list of proportions with a matched list of proportions
> to test for a difference?
>
> So, for example:
>
> list 1 list 2
>
> a1 | b1 a2 | b2
>
> 3 | 4 7 | 9
> 6 | 7 5 | 1
> 9 | 1 3 | 1
>
>
> I want to compare list 1 with list 2 and the samples are matched.
Meaning that
3 4 7 9
are the _counts_ in one stratum of three in all?
And you have an hypothesis that claims the proportions are equal in each
stratum??
The obvious candidate for that setup is a log-linear model for the counts
in a 2 by 2 by 3 table.
See
?loglin
and
?loglm (in MASS)
and the refernces therein.
You can do this type of work in glm() if you understand surrogate Poisson
models as outlined in
McCullagh P. and Nelder, J. A. (1989) Generalized Linear Models. London:
Chapman and Hall.
HTH,
Chuck
> Obviously, I could add the columns and do a binomial test, i.e.
> prop.test(c(18,15),c(30,26)), however, I have a large dataset so this
> would reduce the power of my analysis. I could compare the ratios, i.e.
> a1/(a1+b1) compared to a2/(a2+b2) for the samples in each list, however,
> this does not account for the difference in sample sizes between samples
> in each list.
>
> I have tried a glm where I bind a2 and b2 as the y variable, i.e.
> y<-cbind(a2,b2) and also bind a1 and b1 as the x variable, i.e.
> y<-cbind(a1,b1) and run <-glm(y~x,binomial)
>
> I get this type of output:
>
> Call:
> glm(formula = y ~ x, family = binomial)
>
> Deviance Residuals:
> Min 1Q Median 3Q Max
> -3.20426 -0.72686 -0.01822 0.68320 4.05035
>
> Coefficients:
> Estimate Std. Error z value Pr(>|z|)
> (Intercept) 0.178369 0.186421 0.957 0.339
> xa1 0.008109 0.017430 0.465 0.642
> xb1 -0.026666 0.018153 -1.469 0.142
>
> (Dispersion parameter for binomial family taken to be 1)
>
> Null deviance: 565.14 on 467 degrees of freedom
> Residual deviance: 559.69 on 465 degrees of freedom
> AIC: 1883.3
>
> Number of Fisher Scoring iterations: 3
>
>
> Is this output meaningful? It seems that y is not compared directly with
> x, but rather compared with a1 and b1, which is not intended?
>
> I wonder if this is a suitable approach to the problem? I'll be very
> grateful for any advice or suggestions.
>
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> and provide commented, minimal, self-contained, reproducible code.
>
Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
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