The data presented here were collected as part of a population-based study of sleep disorders, which used a 2-phase protocol in order to recruit participants from various age groups.25–27 In the first phase of the study, a sample of adult men and women (age ≥ 20 years) was randomly selected from local telephone households in 2 counties of Central Pennsylvania (Dauphin and Lebanon) using the Mitofsky-Waksberg 2-stage random digit dialing procedure.28 A within-household selection procedure described by Kish was used to select the specific man or woman to be interviewed.29 Telephone interviews were conducted with 4,364 age-eligible men and 12,219 age-eligible women residing in the sample households, for a total sample of 16,583 with response rates of 73.5% and 74.1%, respectively. The questionnaire employed in this interview included basic demographic and sleep information. In the second phase of this study, a subsample of 741 men and 1,000 women selected randomly from those subjects previously interviewed by telephone were studied in our sleep laboratory. After giving a complete description of the study to the subjects, written informed consent was obtained. The response rates for this phase were 67.8% and 65.8% for men and women, respectively. We contrasted those subjects who were recorded in the laboratory with those who were selected but not recorded in terms of age, BMI, and prevalence of sleep disorders. There were no significant differences between these 2 groups on any of these variables.
Of the 1741 subjects who completed the comprehensive sleep evaluation, 1395 subjects were followed up after an average duration of 7.5 years (mean duration of 4.5 years for women and 10.5 years for men) by one of the investigators (S.C.) via telephone interview. The response rate of the follow-up study was 79.7%. However, if one considers that 215 subjects died between baseline and follow-up, then the response rate of those alive was 90.9%. In the Penn State Cohort Study, men were recruited first and women 5 years later. This explains the 5-year difference in the follow-up period between men and women. After complete description of the follow-up study to the subjects, verbal informed consent was obtained. The whole study procedure was approved by the university's institutional review board.