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Re: st: GEE or svy:logit


From   "Coleman, Brenda" <BColeman@mtsinai.on.ca>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: GEE or svy:logit
Date   Sun, 3 Feb 2008 18:11:53 -0500

Thank you. I very much appreciate your response. I had not considered the
need to weight the observations.
Brenda


----- Original Message -----
From: owner-statalist@hsphsun2.harvard.edu
<owner-statalist@hsphsun2.harvard.edu>
To: statalist@hsphsun2.harvard.edu <statalist@hsphsun2.harvard.edu>
Sent: Sun Feb 03 15:35:26 2008
Subject: Re: st: GEE or svy:logit


 From your description, the household survey is not a formal sample  
of a target population.  (The original case and control groups might  
have been.)  So, off-hand, you do not need any -svy- commands at all,  
ordinary -logit- with a cluster option  or -glm- (which does GEE),  
also with a cluster option, will be sufficient and will give  
equivalent inference.

A more important question is: will the presence of a large percentage  
of case-households distort your analysis?  For example, if one of  
your goals is to predict an outcome and that outcome is related to  
the original case-variable, then it will be over-represented in the  
sample. To get around this, you might want to down-weight the case- 
households.  If case households constitute, say, 1% of the population  
(uncommon disease), then consider giving them a weight of '1' and  
control households a weight of '99'. You can do this in either -glm-  
or -logit-. Again no -svy- version is needed.

However this is extreme,, and it will not work if your controls were  
pair-matched to cases.  If your outcome is unrelated to the outcome  
of the case-control study, then go ahead and use the unweighted  
data.  A convenience sample is a convenience sample.

Steven

> For this study, the individuals are a convenience sample from
> households that participated in a case-control "parent" study in which
> the case households had contaminated water and control households did
> not.
>
> This cross-sectional study includes all of the adults within the case
> and control households who consented to participate by completing the
> individual questionnaire.
>
> Thanks,
> Brenda
>
>
>
> Brenda,  Please give some detail about the survey design: 1. What was
> the target population; 2. How did you select the sample-please give
> all steps.
>
> Steven
> On Feb 2, 2008, at 12:49 PM, b.coleman@utoronto.ca wrote:
>
>> Greetings from a new Statalister.
>> I am in need of advice, including references, if you have any.
>>
>> We have done a household-level cross-sectional survey including  
>> all  consenting adults within the household (1 to 4 per cluster).  
>> There  are both household-level and personal-level variables.
>> The dependent variable is nominal at the personal level (ill/not  
>> ill).
>> The focal independent variable is nomial and at the household  
>> level  (water contaminated/not).
>> Other variables of interest (explanatory, in relation to focal   
>> independent variable) are at the personal and household level.
>>
>> My question is this...do I need to use GEE to adequately account  
>> for  clustering within households OR would the svy:logit in Stata  
>> do  this? (The ICC for illness & household is 0.08, SE 0.08)
>>
>> Regards,
>> Brenda Coleman, PhD candidate
>>
>
>
>
> ----- End forwarded message -----
>
>
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