# [R-sig-eco] Inference, logistic regression

Manuel Spínola mspinola at una.ac.cr
Tue Jun 3 16:19:13 CEST 2008

```Another twist will be to fit models with and without the covariate and
use AICc values, delta AICc, and Akaike weights and interpret parameter
estimates to assess the covariate merits and stay away from P values.
Best,

Manuel Spínola

Andrew Rominger escribió:
> Dear list,
>
> Please pardon this beginner's-level question, I feel it's not quite up
> to the same caliber as recent discussions.
>
> I'm working with a simple logistic regression model comparing the
> presence/absence of an insect species against an index of plant
> species turnover:
>
>> foo<-glm(bout.psol\$pres.de~bout.psol\$index,family=binomial)
>
> The term bout.psol\$pres.de is binary 0,1; and bout.psol\$index is
> continuous.
>
> I'd like to use a likelihood ratio statistic to test the significance
> of this regression, but I'm a little uncertain as how to proceed.
> When I call summary(foo), I get...
>
> Call:
> glm(formula = bout.psol\$pres.de ~ bout.psol\$index,
>     family = binomial)
>
> Deviance Residuals:
>     Min       1Q   Median       3Q      Max
> -1.7180  -1.1289   0.6314   1.0323   1.7499
>
> Coefficients:
>                      Estimate Std. Error z value Pr(>|z|)
> (Intercept)           0.30584    0.23095   1.324  0.18542
> bout.psol.edit\$index  0.04552    0.01439   3.163  0.00156 **
> ---
> Signif. codes:  0 ?***? 0.001 ?**? 0.01 ?*? 0.05 ?.? 0.1 ? ? 1
>
> (Dispersion parameter for binomial family taken to be 1)
>
>     Null deviance: 130.14  on 93  degrees of freedom
> Residual deviance: 118.17  on 92  degrees of freedom
> AIC: 122.17
>
> Taking (Null dev) - (Redid dev), I get 11.97, which I assume to be
> equal to -2log(L,full/L,reduced).  That's the desired test statistic,
> so is it as simple as calling:
>
>> pchisq(11.97,df=92)
> [1] 2.911346e-25          ?
>
> That's an awfully small p-value, I think I'm interpreting something
> wrong.  Any advice would be very welcomed.
>
> Thanks very much in advance
> Andy Rominger
>
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>
>

--
Manuel Spínola, Ph.D.
Instituto Internacional en Conservación y Manejo de Vida Silvestre
Heredia
COSTA RICA
mspinola at una.ac.cr
mspinola10 at gmail.com
Teléfono: 277-3598
Fax: 237-7036

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