From ripley at stats.ox.ac.uk  Tue Apr 24 15:56:55 2012
From: ripley at stats.ox.ac.uk (Prof Brian Ripley)
Date: Tue, 24 Apr 2012 14:56:55 +0100
Subject: [Rd] nobs.glm
In-Reply-To: <CANQBBMsUcKHZeLpN5a1mCnzHhTMNNeSBwiPTUdydc=2jp8netw@mail.gmail.com>
References: <CANQBBMsUcKHZeLpN5a1mCnzHhTMNNeSBwiPTUdydc=2jp8netw@mail.gmail.com>
Message-ID: <4F96B127.7060205@stats.ox.ac.uk>

On 24/04/2012 14:36, Wincent wrote:
> Hi all,
>
> The nobs method  of (MASS:::polr class) takes into account of weight,
> but nobs method of glm does not. I wonder what is the rationale of
> such design behind nobs.glm. Thanks in advance. Best Regards.
>
>> library(MASS)
>> house.plr<- polr(Sat ~ Infl + Type + Cont, weights = Freq, data = housing)
>> house.logit<- glm(I(Sat=='High') ~ Infl + Type + Cont, binomial,weights = Freq, data = housing)
>> nobs(house.plr)
> [1] 1681
>> nobs(house.logit)
> [1] 72
>

Well, the interpretation of 'weights' for a GLM depends on the family. 
They may be equivalent to duplicated observations for a binomial GLM, 
but they are not for a Gaussian one.  The nobs method for class "glm" 
(there is no visible nobs.glm) follows the "lm" method.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
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[Rd] nobs.glm
Prof Brian Ripley