Researchers collected data to examine the relationship between air 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 g/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. In this study,

(a)Identify the cases (sample) in this study

(b)Identify the variables in the study and their (categorical or numeric). If numeric, state if they are continuous or discrete. If categorical, say if they are ordinal or nominal

(c)Identify the main research question of the study

(d)What type of study is this? (experiment or observational study)

(e)If this is an observational study, can you specify what type of observational study this is?

(f)Can you think of one potential confound variable not collected in this study?

(g)Can you think of one blocking variable not collected in this study?