Random-digit dialing (RDD) is a method used to select participants for telephone surveys and for related purposes such as selecting control group subjects in casecontrol studies. The basis of RDD is the random generation of telephone numbers that are used to contact potential survey respondents or study participants. Several major U.S. Federal Government public health surveillance projects use RDD, including the Behavioral Risk Factor Surveillance System (BRFSS) and the National Immunization Survey (NIS). RDD does not require the use of telephone directories and has the advantage of including as potential respondents households with unlisted numbers or who have recently moved or changed phone service; failure to include these types of households can seriously bias the sample. However, RDD has the disadvantage that many of the ...