    st: Different results from svy: tab vs. svy: prop

 From To Subject st: Different results from svy: tab vs. svy: prop Date Tue, 24 Oct 2006 08:04:28 -0600

Friends,

I'm comparing tobacco survey data for two years. I want to standardize
estimates for the later year (2005) using age-sex-ethnicity distribution
of the earlier year (2001). When I run the estimates separately using -
svy: prop - I get the expected results: std-ized and non-std-ized
estimates are essentially the same for 2001 but different for 2005.
However, when I run the contrast directly using - svy: tab - the
std-ized estimates change by 0.1% for 2001 (the reference year for
std-ized weights) but don't change for 2005. Can someone explain why
this happens? Here's the output.

thx...arnold

/* estimated smoking prevalence is same for 2001 data using
standardization to 2001 sex-age-ethnicity*/

. svy: prop nowsmoke if year==2001, stdize(stdstrata)
stdweight(stdwgt01)
(running proportion on estimation sample)
Survey: Proportion estimation
Number of strata =       8          Number of obs    =   12964
Number of PSUs   =   11219          Population size  = 3.1e+06
N. of std strata =      48          Design df        =   11211
Linearized         Binomial Wald
Proportion   Std. Err.     [95% Conf. Interval]
nowsmoke
0    .8026479   .0043218      .7941765    .8111193
1    .1973521   .0043218      .1888807    .2058235

. svy: prop nowsmoke if year==2001,
(running proportion on estimation sample)
Survey: Proportion estimation
Number of strata =       8          Number of obs    =   12964
Number of PSUs   =   11219          Population size  = 3.1e+06
Design df        =   11211
Linearized         Binomial Wald
Proportion   Std. Err.     [95% Conf. Interval]
nowsmoke
0    .8026311   .0043816      .7940423    .8112199
1    .1973689   .0043816      .1887801    .2059577

/* estimated smoking prevalence is different (as expected) for 2005 data
using standardization to 2001 sex-age-ethnicity*/

. svy: prop nowsmoke if year==2005, stdize(stdstrata)
stdweight(stdwgt01)
(running proportion on estimation sample)
Survey: Proportion estimation
Number of strata =      65          Number of obs    =   12086
Number of PSUs   =   11440          Population size  = 3.4e+06
N. of std strata =      48          Design df        =   11375
Linearized         Binomial Wald
Proportion   Std. Err.     [95% Conf. Interval]
nowsmoke
0    .8254121   .0050684      .8154771    .8353471
1    .1745879   .0050684      .1646529    .1845229

. svy: prop nowsmoke if year==2005,
(running proportion on estimation sample)
Survey: Proportion estimation
Number of strata =      65          Number of obs    =   12086
Number of PSUs   =   11440          Population size  = 3.4e+06
Design df        =   11375
Linearized         Binomial Wald
Proportion   Std. Err.     [95% Conf. Interval]
nowsmoke
0    .8267205   .0050774      .8167679    .8366731
1    .1732795   .0050774      .1633269    .1832321

/* using tab, standardized difference appears in 2001 data, not 2005
data*/

. svy: tab nowsmoke year, col
(running tabulate on estimation sample)
Number of strata   =        73        Number of obs      =       25050
Number of PSUs     =     22659   Population size    =        6480351.9
Design df          =           22586
year
nowsmoke   2001   2005  Total
0  .8026  .8267  .8152
1  .1974  .1733  .1848
Total      1      1      1
Key:  column proportions
Pearson:
Uncorrected   chi2(1)         =   24.0803
Design-based  F(1, 22586)     =   12.6389            P = 0.0004

. svy: tab nowsmoke year, col stdize(stdstrata)    stdweight(stdwgt01)
(running tabulate on estimation sample)
Number of strata   =        73        Number of obs      =       25050
Number of PSUs     =     22659   Population size    =        6480351.9
N. of std strata   =        48          Design df          =
22586
year
nowsmoke   2001   2005  Total
0  .8017  .8263  .8144
1  .1983  .1737  .1856
Total      1      1      1
Key:  column proportions
Pearson:
Uncorrected   chi2(1)         =   24.9490
Design-based  F(1, 22586)     =   13.2600            P = 0.0003

Arnold H. Levinson, Ph.D.
Director, Tobacco Program Evaluation Group (TPEG)
Assistant Professor of Preventive Medicine
University of Colorado at Denver & Health Sciences Center
AMC Cancer Research Center
1600 Pierce Street, Lakewood CO 80214
Arnold.Levinson@uchsc.edu
303-239-3402

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