Factors influencing outcomes in cystic fibrosis: a center-based analysis. - Johnson C; Butler SM; Konstan MW; Morgan W; Wohl MEB; Scientific Advisory Group and the Investigators of the Epidemiologic Study of Cystic Fibrosis; Johnson, Charles; Butler, Steven M; Konstan, Michael W; Morgan, Wayne; Wohl, Mary Ellen B (CHEST, 2003)