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st: Standard error of the weighted mean


From   nick bungy <nickbungystata@hotmail.co.uk>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Standard error of the weighted mean
Date   Fri, 3 May 2013 18:03:57 +0100

Dear Statalist,

I am having issue finding the correct way to calculate the standard error of a probability weighted mean. I can successfully calculate the sample standard deviation, as taken from:
 
http://www.stata.com/support/faqs/statistics/weights-and-summary-statistics/
 
s2 = {n/[W(n - 1)]} sum wi (xi - xbar)2
 
The Stata article suggests that the standard error of the weighted mean is then simply:
 
V_srs = s2/n
 
Where V_srs is an ‘estimate of the variance of muhat [mean] assuming a simple random sample of the same number of observations’
 
However, when I calculate V_srs and compare it to the linearized standard error as taken from the svy: commands, the linearized standard error is significantly larger.
 
Since my calculations and Stata agree on s^2, presumably I must be calculating V_srs (variance of the mean) incorrectly.
 
Can anyone shed some light on where I am going wrong? Here's an example of the disparity between my mata code and the official svy command.
 
sysuse auto
gen pweight = trunk^2 + 1
 
mata
st_view(y=. ,. , "mpg")
st_view(weight =. ,., "pweight")
 
Weighted_mean = colsum(y:*weight) / colsum(weight)
Weighted_var = rows(y)/(colsum(weight)*(rows(y)-1))*colsum(weight:*(y :- Weighted_mean):^2)
Weighted_sd = sqrt(Weighted_var)
SD_Mean = sqrt(Weighted_var / rows(y))
 
Weighted_sd
SD_Mean
 
end
 
svyset [pweight = pweight]
svy: mean mpg
estat sd
 
Thanks 		 	   		  
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