In the NPHS, households were selected though a stratified, multilevel cluster sampling of private residences using provinces and/or local planning regions as the primary sampling unit.
The MP investigation (1994-1997) was the largest pesticide misuse case in the nation, ultimately involving the sampling of 9,000 residences and the decontamination of 1,000 properties.
Briefly, we used an added-digit technique [ 12] to identify, via telephone, a sample of 156 residences with children in regions of Washington State with varying levels of arsenic and nitrate.
Samples of participant residences in each metropolitan region were selected for monitoring as home outdoor sites on a rotating basis, with most locations monitored one to three times in different seasons.
Specifically, in the first stage a random sample of 84 blocks of residences were selected according to information from the 1991 national census.
For every pre-patient, four control subjects were randomly selected from the same cohorts and matched for sex, age at the time of blood sampling, and area of residence.
During a local health fair conducted in the towns of Afton, Thayne, and Alpine during May 1999, participants donated 5 mL of blood during phlebotomy for screening to detect hyperlipidemia and answered a questionnaire regarding age, gender, treated versus nontreated domestic water supply, history of bloody diarrhea, location of sampling, and town of residence.
The water department later told the Guardian that the latest round of water sampling included 24 residences of employees, of the total 51 sites sampled.
As explained in the Methods section, owing to the lack of information on 168 households, 1413 sample units (residences of the subjects of the study) were georeferenced, and not 1581.
Given the absence of geographic identifiers for the mobile numbers, no quota was applied to the mobile sample and state of residence was established on contact.
