Val wrote:
Hi all
Assume I have a data set xx;
Group: 1=group1 , 2=group2
IQ: 1= High, 0 =low
fit <- glm(IQ ~group, data = xx, family = binomial())
summary(fit)
Results
Estimate Std. Error z value Pr(>|z|)
(Intercept) -2.55456 0.210 -12.273 < 5e-16 ***
group 0.36180 0.076 3.952 5.24e-05 ***
the odd ratio = exp(0.36180 )= 1.435912
My question is that the log-odd estimate 0.3618 is it for group1 or group2?
What does the odd ratio 1.43359 is interpreted?
Val,
Before using R's model fitting functions, it helps
to understand your model. See any introductory text
on logistic regression.
Despite what you claim, it appears that your data 'set'
may be a data.frame with variables 'IQ' and 'group',
something like this:
set.seed(34)
xx <- data.frame(IQ = sample(0:1, 10, TRUE), group = gl(2, 5))
xx
The summary you show was not produced by R, at least
not as you show it. Here's the result for the above
data:
fit <- glm(IQ ~ group, data=xx, family=binomial())
summary(fit)
## snipped R output
Coefficients:
Estimate Std. Error z value Pr(>|z|)
(Intercept) -0.4055 0.9129 -0.444 0.657
group2 0.8109 1.2910 0.628 0.530
## end R output
Note the '2' in 'group2'. R is smart. It let's you
know which level of factor 'group' should get the
added 0.8109 in its log-odds estimate. From your
reported output it would be impossible to tell that.
You might have set 'group' to have level '2' as the
reference level, in which case R would show a
'group1' row.
For more on logistic regression, you could consult
Wikipedia, but here's a brief explanation of your
simple case:
Consider two models, one for each group:
log(Pr(IQ=1)/Pr(IQ=0)) = const_1 (group 1)
log(Pr(IQ=1)/Pr(IQ=0)) = const_2 (group 2)
Combine these into a single model, using an
indicator variable to signal the group:
log(Pr(IQ=1)/Pr(IQ=0)) = beta_0 + beta_1 * Indic(group 2)
where Indic(group 2) = 1 for group 2 and 0 otherwise and
beta_0 = const_1,
beta_0 + beta_1 = const_2.
This should help you answer your questions yourself.
- Peter Ehlers
Thanks in advance
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--
Peter Ehlers
University of Calgary
______________________________________________
R-help@r-project.org mailing list
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
