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Re: st: Unexpected proportions after survey commands


From   sjsamuels@gmail.com
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Unexpected proportions after survey commands
Date   Sat, 9 May 2009 20:20:13 -0400

A probability weight is the number of people represented by those in a
sample member.  Your weights look nothing like numbers of people.  In
your first sample, the HH probability weights (before non-response
adjustments) should be 10.0, because you took a 10% sample of HH.  If
you interviewed every adult in the HH, they retain the HH weight.  If
you interviewed 1/K in a household, the person weight is the HH weight
x K.

It's not clear whether your frame of tourist workers (sample 2)  was
of HH or people.  If people, then you should be interviewing only
people who work in tourism, not their HH members--as HH members would
not have been in the frame.  Since I don't know your sampling scheme,
I don't know how to compute the sampling weight.

When you have 2 samples, as you did here, treat each one as coming
from a different stratum.  Transfer the people in sample who work in
tourism to the 2nd stratum, and retain their original sampling weight.

If villages are strata, then you have 2x10 = 20 sampling strata.
However it sounds like 10 villages are themselves a convenience
sample.  If so, then keep the two samples as strata.  Your PSU should
probably be HH.  However if you interviewed only one person per HH,
then PSU can be  person.

After computing the sampling weights, you can,  as Michael states, use
the -poststratify- option in Stata to reproduce the tourism counts.
Your post-stratification totals (tourism workers, non-tourism workers,
should add to the estimated population totals in the 10 villages;
0.84% should be tourism workers, and 98.26% should be non-tourism
workers.   If you want separate estimates of impact in each village,
then you can use the the villages to also define your post-strata: 10
villages x 2 tourist-worker-status strata.

Finally, unless one goal is to compare tourism and non-tourism
workers, it was not necessary to enhance your sample with tourism
workers.   Tourism workers are obviously greatly affected by tourism,
compared to non-tourism workers.  However, they constitute only 0.84%
of the population,  so contribute minimally to the overall effects of
tourism on the population.

if you need further assistance, the University of Florida has a number
of faculty with experience in survey sampling.

-Steve



On Sat, May 9, 2009 at 5:13 PM, Jean-Gael Collomb <JG@ufl.edu> wrote:
> Hello all,
>
> I have a question about using post stratification weights and using Stata's
> survey commands. After setting the weights, I do not get the proportions I
> expected.
>
> My overall research question is to see if tourism (TOURIND) influences
> quality of life in several communities in a rural province of Namibia. My
> aim was to conduct individual interviews in a sample of 10% of all
> households in each community. I obtained household census counts from key
> informants within the community and my own double checks during field work.
>  This random sample yielded a random sample of 395 interviews, of which only
> 9 (2.3%) were conducted with individuals working in tourism. Given this very
> low number of respondents who worked in tourism and my interest in trying to
> understand the impact of tourism, I established a sampling frame restricted
> to individuals working in tourism and interviewed 72 individuals. [Two of
> those interviews were conducted with individuals not employed in tourism but
> living in a household where someone was]. In total, I thus interviewed 467
> people, among which 79 worked in tourism. My full sample oversampled tourism
> employees and i think it would be wrong to derive from it that 17%
> (79/467*100) of the population works in tourism. I think Post stratification
> weights should be assigned to my data set to correct for the oversampling.
> In fact, the percentage of the population working in tourism varies by
> communities and thus different weights should be calculated for different
> communities. I used existing reports documenting total numbers of community
> residents employed by local tourism operators and total population size as a
> basis to calculate the "true" distribution of tourism employees (weight2).
> The weights were calculated by dividing the “true” percentage by the
> “oversampled” percentage.
>
> The problem is that when I apply the weights in Stata, I do not get the
> proportion I expected. Specifically, I expected that after svyset _n
> [pweight = samplewt2] and svy: tab tourind, I would find that 0.84% of the
> population could be labeled TOURIND, but Stata returns a value of 3.25% (and
> similar discrepancies for each community).
>
> I am not sure I am doing something wrong in calculating the weights,
> assigning the weights to my dataset, or entering the tab commands in svy
> mode. I’d greatly appreciate your help in helping move past this and take
> advantage of survey commands in Stata.
>
> Thank you very much if you have time to give me some feedback or point me
> towards the best information source (textbook?).
>
> Cheers,
>
> Jean-Gael Collomb, jg@ufl.edu
>
> (PS. I run Stata 10 in Mac OSX)
>
>
>
> State code entered:
>
> *ASSIGNING POST STRATIFICATION WEIGHTS
>
> *-------------------------------------
>
> gen samplewt2=0
>
> label var samplewt2 "Post Stratification sample weight 2"
>
> replace samplewt2=0.99975204562360500 if conservancy==1 & sample==1
>
> replace samplewt2=0.04357333333333330 if conservancy==2 & sample==2
>
> replace samplewt2=1.39197814207650000 if conservancy==2 & sample==1
>
> replace samplewt2=0.10144078144078100 if conservancy==3 & sample==2
>
> replace samplewt2=1.18320139407518000 if conservancy==3 & sample==1
>
> replace samplewt2=0.05683908045977010 if conservancy==4 & sample==2
>
> replace samplewt2=1.47985380116959000 if conservancy==4 & sample==1
>
> replace samplewt2=0.01906976744186050 if conservancy==5 & sample==2
>
> replace samplewt2=1.05030411449016000 if conservancy==5 & sample==1
>
> tab tourind
>
> bysort conservancy: tab tourind
>
> *applying weight2 (those derived from IRDNC data)
>
> svyset _n [pweight = samplewt2]
>
> svy: tab tourind, percent
>
>
>
> Jean-Gael "JG" Collomb
>
> PhD candidate
>
> School of Natural Resources and Environment / School of Forest Resources and
> Conservation
>
> University of Florida
>
> jgcollomb@gmail.com
>
> jg@ufl.edu
>
> +1 (352) 870 6696
>
>
>
>
>
> *
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>

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