[R] puzzling results from logistic regression
Peter Ehlers
ehlers at ucalgary.ca
Thu Mar 1 00:55:31 CET 2012
Michael (OP),
Not that it's necessary, but you might also find
confint(aa)
to be instructive.
Peter Ehlers
On 2012-02-29 07:20, R. Michael Weylandt wrote:
> Formally, look at Pr(>|z|). Informally, look at the null and residual
> deviances from print(aa).
>
> Michael
>
> On Wed, Feb 29, 2012 at 10:14 AM, Michael<comtech.usa at gmail.com> wrote:
>> How did you see it's non-significant?
>>
>> Thanks!
>>
>> On Wed, Feb 29, 2012 at 9:10 AM, Sarah Goslee<sarah.goslee at gmail.com>wrote:
>>
>>> On Wed, Feb 29, 2012 at 10:02 AM, Michael<comtech.usa at gmail.com> wrote:
>>>> Hi all,
>>>>
>>>> As you can see from below, the result is strange...
>>>
>>> Not really.
>>>
>>>> I would imagined that the bb result should be much higher and close to 1,
>>>> any way to improve the fit?
>>>>
>>>> Any other classification methods?
>>>>
>>>> Thank you!
>>>>
>>>> data=data.frame(y=rep(c(0, 1), times=100), x=1:200)
>>>> aa=glm(y~x, data=data, family=binomial(link="logit"))
>>>>
>>>> newdata=data.frame(x=6, y=100)
>>>> bb=predict(aa, newdata=newdata, type="response")
>>>> bb
>>>>
>>>>
>>>>> bb
>>>>
>>>> 1
>>>>
>>>> 0.4929125
>>>
>>>
>>> What did you expect? Your model is completely nonsignificant; there's no
>>> way to predict y from x, and that's what your predicted value tells you.
>>>
>>>> summary(aa)
>>>
>>> Call:
>>> glm(formula = y ~ x, family = binomial(link = "logit"), data = data)
>>>
>>> Deviance Residuals:
>>> Min 1Q Median 3Q Max
>>> -1.190 -1.177 0.000 1.177 1.190
>>>
>>> Coefficients:
>>> Estimate Std. Error z value Pr(>|z|)
>>> (Intercept) -0.030152 0.283924 -0.106 0.915
>>> x 0.000300 0.002450 0.122 0.903
>>>
>>> (Dispersion parameter for binomial family taken to be 1)
>>>
>>> Null deviance: 277.26 on 199 degrees of freedom
>>> Residual deviance: 277.24 on 198 degrees of freedom
>>> AIC: 281.24
>>>
>>> Number of Fisher Scoring iterations: 3
>>>
>>>
>>> I can only assume that you didn't construct the data frame that
>>> you intended to test.
>>>
>>> --
>>> Sarah Goslee
>>> http://www.functionaldiversity.org
>>>
>>
>> [[alternative HTML version deleted]]
>>
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>
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