To determine whether the risk of HIV seroconversion among daily smokers of crack cocaine changed over time , we used Cox proportional hazards regression and divided the study into 3 periods : May 1 , 1996-Nov. 30 , 1999 ( period 1 ) , Dec. 1 , 1999-Nov. 30 , 2002 ( period 2 ) , and Dec. 1 , 2002-Dec. 30 , 2005 ( period 3 ) .
O O O O O O B-DiseaseOrPhenotypicFeature I-DiseaseOrPhenotypicFeature O O O O B-ChemicalEntity I-ChemicalEntity O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O