Table 2.

Baseline characteristics of the study population in the original cohort (OC) and the outcome-dependent sample (ODS). The latter is a subset of the former, and binary covariates are shown as proportions. There were 2 cohorts in this study sample, fourth graders and seventh graders. We show the proportion in the seventh grade. We also display the 25th, 50th, and 75th percentiles of the 4-day average ozone concentrations across all observed values

	OC	ODS
Number of subjects	1286	682
Number of observations	5341	2886
Proportion asthmatic at baseline	0.23	0.24
Proportion male	0.50	0.44
Proportion not white	0.15	0.14
Proportion with secondhand smoke exposure	0.38	0.36
Proportion in grade 7	0.34	0.37
Ozone concentrations (ppb)	(35, 46, 61)	(34, 44, 58)