Prevalence of sleep apnea syndrome--estimation by two stage sampling

This article describes stepwise the methodological and statistical considerations made in the planning of an epidemiological survey of the prevalence of the sleep apnea syndrome (SAS) in the municipality of Uppsala in Sweden. The investigation had to be confined to 60 subjects, since all-night polysomnographic studies are required for an unequivocal diagnosis of SAS. It was decided to investigate men 30 to 69 years old. Initially, the possibility of taking a simple random sample (SRS) was considered, but statistical calculations showed that for prevalences between 1-3% this would lead to totally unacceptable results. A postal questionnaire, sent to the total population of 35,779 men in this age group, was then considered and, depending on their replies, they would be divided into low-risk and high-risk stratums of SAS. Optimal numbers would then be called from each group for polysomnographic studies. This also proved impossible, as the lowest possible standard error was still too large and the samples would contain unacceptably few cases of SAS. We therefore decided to concentrate on the highrisk stratum, obtaining an estimated under limit of the prevalence. For economical reasons, we could not send a questionnaire to all the 35,779 individuals, but based the investigation on a SRS of 4,000 men, post-stratified in a high-risk and a low-risk group. From the high-risk group, 60 men were then selected for polysomnographic studies.