[R] checking the significance of the effects of altitude on endemic species richness

spyros stsiftsi at bio.auth.gr
Tue Jan 29 12:52:05 CET 2013

Hi all,
I run a glm analysis of the effects of altitude to the proportion of endemic
plant species of Crete and the results are:

glm(formula = pCRENDSR ~ Alt, family = binomial, data = dat)

Deviance Residuals: 
      Min         1Q     Median         3Q        Max  
-0.078121  -0.022688   0.005328   0.027674   0.056688  

              Estimate Std. Error z value Pr(>|z|)  
(Intercept) -3.2005908  1.5129560  -2.115   0.0344 *
Alt          0.0012146  0.0008905   1.364   0.1726  
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 

(Dispersion parameter for binomial family taken to be 1)

    Null deviance: 2.217584  on 23  degrees of freedom
Residual deviance: 0.029835  on 22  degrees of freedom
AIC: 14.469

Although there is an increase to the proportion of the endemic species with
altitude, the results of the glm do not show a significant effect. However,
in absolute values, the number of endemic species is significantly increased
with altitude. I would like to ask if I have to do something else for the
statistical significance except from the p=0.1726.
Thank you in advance,

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