The sample was selected using a stratified multistage
cluster technique. In the first stage, the sample consisted of 34
primary sampling units (PSUs). In July 1975, 24 additional PSUs were
added. However, the increased sample size is not reflected in the data
until July 1976. In the second stage, sample segments were chosen from
the PSUs at a rate of 1 in 24. Note that one PSU has approximately 600
segments. Approximately 50,000 establishments were selected for
interview.
Commercial establishments in the United States in
1973-1977.
personal interviews and telephone interviews
survey data