One can always use the delta method (statistical differentials) for 
problem of that sort, but in case the idea is to compute a confidence 
interval, intervals based on asymptotic normality should be computed in 
the scale where normality is most plausible, likely logit in this case, 
and the bounds transformed as needed. 

r-sig-ecology-bounces@r-project.org wrote on 07/08/2010 07:52:14 AM:

> From:
> 
> Manuel Spínola <mspinola10@gmail.com>
> 
> To:
> 
> r-sig-ecology@r-project.org
> 
> Date:
> 
> 07/08/2010 07:56 AM
> 
> Subject:
> 
> [R-sig-eco] Transforming logits to probabilities
> 
> Dear list members,
> 
> I am using function invlogit from the "arm" package to get the 
> difference in probabilities for a explanatory factor from the model 
> below (I am working with factor "iluminacion").
> 
>  > invlogit(-0.4241 - 0.4804 * 0) - invlogit(-0.4241 - 0.4804 * 1) # 
> difference in probability for iluminacion
> [1] 0.1074094
> 
> How can I transform the standard error of the difference from logit to 
> probability?
> 
> The standard error in logits is 0.2416 (taken from the model below).
> 
> Best,
> 
> Manuel
> 
> 
>  > summary(mod6)
> 
> Call:
> glm(formula = condicion ~ iluminacion + animales + cielo.raso +
>      piso + paredes, family = binomial, data = reglog)
> 
> Deviance Residuals:
>      Min       1Q   Median       3Q      Max
> -1.2527  -1.0034  -0.8276   1.2208   1.6981
> 
> Coefficients:
>                   Estimate Std. Error z value Pr(>|z|)
> (Intercept)       -0.4241     0.2833  -1.497   0.1343
> iluminacion[T.1]  -0.4804     0.2416  -1.988   0.0468 *
> animales[T.1]      0.4613     0.2385   1.934   0.0531 .
> cielo.raso[T.1]   -0.1386     0.2840  -0.488   0.6254
> piso[T.1]          0.1380     0.2934   0.470   0.6382
> paredes[T.1]      -0.1289     0.3012  -0.428   0.6687
> ---
> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
> 
> (Dispersion parameter for binomial family taken to be 1)
> 
>      Null deviance: 433.04  on 319  degrees of freedom
> Residual deviance: 424.30  on 314  degrees of freedom
>    (25 observations deleted due to missingness)
> AIC: 436.3
> 
> Number of Fisher Scoring iterations: 4
> 
> 
> -- 
> Manuel Spínola, Ph.D.
> Instituto Internacional en Conservación y Manejo de Vida Silvestre
> Universidad Nacional
> Apartado 1350-3000
> Heredia
> COSTA RICA
> mspinola@una.ac.cr
> mspinola10@gmail.com
> Teléfono: (506) 2277-3598
> Fax: (506) 2237-7036
> 
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> R-sig-ecology@r-project.org
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