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-boun...@r-project.org wrote on 07/08/2010 07:52:14 AM:
From:
Manuel Spínola mspinol...@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
mspin...@una.ac.cr
mspinol...@gmail.com
Teléfono: (506) 2277-3598
Fax: (506) 2237-7036
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