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RE: st: RE: Survey design degrees of freedom help


From   <Andrew.Clapson@statcan.gc.ca>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: Survey design degrees of freedom help
Date   Thu, 3 Sep 2009 15:00:16 -0400

Jennifer,

Off the top of my head, it seems to me that, if degrees of freedom can
be considered roughly as the 'independent pieces of information' in the
model, I suppose that given the random choosing of your clusters
(villages), those 20 sub-groups are your only independent pieces of
information in your sample, and anything selected out of that
(sub-villages, then households) would be related, as they are in the
same cluster or PSU. 

BUT, given the further sampling of households out of 'sub-villages' -
isn't this multi-stage sampling?  Stata can handle that as well, I
believe, although I haven't had cause to use it (yet).  I am not sure
how that would affect the degrees of freedom as Stata calculates it.

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Jennifer
Schmitt
Sent: September 3, 2009 1:48 PM
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: RE: Survey design degrees of freedom help


My sample size is 489, but I only have 20 PSU, 3 Strata so my design df 
= 20-3-1 (for the constant).  I have a stratified, clustered sample and 
so my design df are based off my PSU and are thus really low.  I just do

not know know why this is the case (I have accepted that it is the 
case), but I can't defend that without knowing why (or maybe it is 
wrong, but everything in the FAQ and help sections online suggest it is 
correct).  Thanks.

Andrew.Clapson@statcan.gc.ca wrote:
> Your degrees of freedom are 16??
> What is your sample size?
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Jennifer 
> Schmitt
> Sent: September 3, 2009 11:26 AM
> To: statalist@hsphsun2.harvard.edu
> Subject: st: Survey design degrees of freedom help
>
>
> Hello everyone,
> I need some important clarification about the design degrees of 
> freedom
> for stratified clustered survey analysis.  I have data that was 
> stratified by three areas (NW, SW, and E).  We randomly chose villages

> from these three areas, then chose 5 subvillages within the village
and 
> within the subvillage we chose households (the unit of interest for my

> analysis).  I am running logistic regressions with PSU = village,
strata
>
> = area and probability weights.  My design degrees of freedom are 16
> (PSU-strata-one for the constant term).  I get that.  What I do not 
> understand is WHY and how to explain to others unfamiliar with STATA 
> that it is correct, any answers to this would be greatly appreciated.

> The reason this is an issue is that I want to test more than 16 
> variables at once and obviously I can't with only 16 df.  Thank you.
> Jennifer
>
>   

-- 
Jennifer Schmitt
PhD Candidate - Conservation Biology Program
jorg0206@umn.edu

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