Re: st: Two different sampling strategies within the same survey

[Steven Samuels <sjsamuels@gmail.com>]

Etan-

The -svyset- statement is not correct, nor is the count of stages (sub- 
sampling of women within a household adds a stage). Based on the  
information you've provided, I suggest the following:

Let urban_rural be the variable that designates urban or rural. Define  
a new variable "psu". In the rural areas, psu = village ID. In the  
urban areas it is electoral ward. Then use the following -svyset-  
statement.

svyset psu [pweight= final person weight], strata(urban_rural)

Some thoughts:

Why only the one stage of sampling?  Only one census tract was  
selected in each ward, so variance due to sampling of tracts is  
incorporated into the between-ward (psu) component of variance. Adding  
the HH and person sampling stages to -svyset- would not change stand  
errors by much (not at all, for some designs) and would not be  
accurate, because Stata assumes that there is simple random sampling  
at later stages.  In fact, many published data files from multi-stage  
samples take a similar -svyset- command, because they identify only  
the strata and primary sampling units.

For analyzing individual responses, use the person weight, not the  
household weight. The two are not the same, because women were sampled  
within households.

If one purpose of your study is to estimate descriptive statistics  
such as means or proportions and if the fractions of villages and  
wards selected were large, then include the fpc option in your - 
svyset- statement. When you do hypothesis testing and modeling, then  
omit the fpc in a second -svyset- statement.

You give no details about how villages or electoral wards were  
selected. If any were selected with certainty-not sampled, but chosen  
ahead of time, designate these as "certainty" units in the -svyset-  
command.

Use correct terminology.  I am guessing that you mean that when you  
say "systematic selection", you mean "systematic sampling".  The term   
"selected" for PSUs and census blocks conveys no information about the  
sampling procedure.

If you are uncertain about how to proceed, I suggest that you read the  
study documentation carefully and, if necessary, contact the study  
statistician. I also suggest you read a good sampling text, such as  
Sharon Lohr's Sampling: Design and Analysis.  If you are familiar with  
survey concepts, the Stata 11 Manual entry for -svyset- is helpful for  
explaining the options in detail.

Steve

Steven J. Samuels
sjsamuels@gmail.com
18 Cantine's Island
Saugerties NY 12477
USA
Voice: 845-246-0774
Fax: 206-202-4783






On Nov 30, 2010, at 9:37 PM, Etan Lakam wrote:

Dear Listers,

I am grappling with how to set the sampling design in Stata using the
svyset command. My problem is that within the same survey a two-stage  
design
was used in rural areas and a three-stage design in urban areas.

For the the two-stage design: villages were selected and then there  
was a
systematic selection of households. For the three-stage design:  
electoral
wards were selected, then a census block was taken from each ward and  
then
household were selected and then women were systematically selected  
within
each household.

I have two queries:
1- Would the following command for setting the sampling design in the
urban area be correct?
*svyset electoral || census || household [pweight=house]*
where electoral is the variable for electoral ward, census is the  
variable
for census block and household is the variable for household, house is  
the
household weight.

2- How do I account for the two sampling strategies as I would like to
analyse the whole data: urban+rural? In other words, how do I set the
overall sampling design using svyset?

Thank you in advance

Etan

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