The primary sampling units, strata, and sampling weights were taken into account using STATA release 11.0 survey-related commands.
To account for the complex NHANES survey design, sampling weights, strata, and primary sampling units were adjusted in all analyses as recommended by the NCHS.
NCHS-created sampling weights, strata, and primary sampling units were applied in all statistical analyses according to the NCHS guidelines to yield robust SEs and unbiased point estimates, and to account for the complex, stratified multistage probability sample design of NHANES.
In the first stage, the total sample frame will be divided into six strata and representative samples of urban and rural EAs will be selected from these strata to obtain the primary sampling unit (PSU).
We used STATA 10 svy commands (STATA Corporation, College Station, TX) for data analysis by incorporating probability sampling weights in conjunction with strata and primary sampling units (psu) generated by NHIS complex survey design.
To account for design effects associated with the complex sample design, data were analysed using survey procedures that adjusted for strata and primary sampling units [ 12].
The proc rlogist command in SUDANN allowed us to specify strata and primary sampling unit (PSU) variables to account for clustering related to the hierarchical sampling structure, and allowed us to specify weight variables to account for the oversampling of certain groups [ 12].
The Survey command in STATA was used to declare the strata and primary sampling unit.
Calculations were adjusted for complex survey design using strata and primary sampling units provided by the NCHS, along with survey weights.
The standard errors are slightly different; the standard error from the LOGISTIC procedure is less than the standard error from the SURVEYLOGISTIC procedure, because the latter uses a sandwich-type robust estimator to account for strata and the sampling proportion.
