Most survey designs over sample certain minority populations, which means that weighting will need to be used in analyses to give accurate results. There may also be other components of the design of the study that must be accounted for in the analysis. This information (and code) is usually found in handbooks and online, and needs to be included in the analysis of certain data sets. http://www.cdc.gov/nchs/tutorials/Nhanes/SurveyDesign/Weighting/intro.htm