Questionnaire Dwelling Unit-Level and Person Pair-Level Sampling ...

2. Questionnaire Dwelling Unit and Pair
Selection Probabilities
Similar to the 1999–2001 National Household Surveys on Drug Abuse (NHSDAs) and
the 2002–2005 National Surveys on Drug Use and Health (NSDUHs), 6 the 2006 NSDUH had a
two-phase design and used a computer-assisted interviewing (CAI) method. There were four
stages of selection: census tracts, segments within census tracts, dwelling units (DUs) within
segments, and persons within dwelling units. Any two survey eligible persons had some nonzero
chance of being selected and, when both were selected, they formed a within household pair.
This design feature is of interest to NSDUH researchers because, for example, it allows analysts
to examine how the drug use propensity of an individual (in a family) relates to the drug use
propensity of other members residing in the same dwelling unit (Morton, Chromy, Hunter, &
Martin, 2007).
For the 1999–2001 surveys, the method used for selecting pairs was as follows. For a
given DU, if the sum of the age-specific selection probabilities was larger than 2, then the
individual person-selection probabilities were ratio adjusted downward to make the sum equal to
2. If the sum was less than 2, the difference between 2 and the sum of the probabilities was
evenly distributed over 3 dummy persons so that the sum of the person probabilities was made to
equal 2. Brewer's method then was applied to select a person pair using the pair selection
formula (2.1). If the selected pair consisted of two real persons, then both persons were selected.
If the selected pair consisted of one real person and one dummy person, then the real person was
selected. If the selected pair consisted of two dummy persons, no one was selected from that DU.
The 2006 survey pair-sampling algorithm was the same as that used in 2002–2005, which
was modified from the one used for the previous surveys (1999–2001) to increase the number of
pairs selected in the sample, as explained below. For dwelling units with the sum of person-level
selection probabilities (denoted by S) less than 2, the earlier algorithm was modified (see Case II
below) to increase the chance for selecting a pair. However, for dwelling units with S ≥ 2, there
was no need to change the algorithm denoted below by Case I. A summary of sample size by
pair age and other domains is provided in Table 2.1.
2.1 Pair Selection Probability
Case I: DUs with S ≥ 2
For a given DU, if the sum of the age-specific person selection probabilities (S) was
larger than 2, then the selection probability was ratio adjusted by a multiplicative adjustment
factor so that all probabilities were scaled down to sum to exactly 2. Now, Brewer's method sets
the pairwise selection probabilities at
6 This report presents information from the 2006 National Survey on Drug Use and Health (NSDUH). Prior
to 2002, the survey was called the National Household Survey on Drug Abuse (NHSDA).
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