Researchers collected data to examine the relationship between pollutants and preterm births in Southern California. During the study air pollution levels were measured by air quality monitoring stations. Specifically, levels of carbon monoxide were recorded in parts per million, nitrogen dioxide and ozone in parts per hundred million, and coarse particulate matter PM10 in μμg/m3. Length of gestation data were collected on 143,196 births between the years 1989 and 1993, and air pollution exposure during gestation was calculated for each birth. The analysis suggested that increased ambient PM10 and, to a lesser degree, CO concentrations may be associated with the occurrence of preterm births. a. The paragraph above describes an :.
A. Observational Study
B. Experiment
b. The cases in this research are: .
A. Each birth between 1989 and 1993
B. Each type of pollution measured by air quality monitoring stations
C. Each year between 1989 and 1993
D. Air pollution exposure during gestation
c. What is the research question?