Cigarette smoking and tobacco chewing in relation to hypopharyngeal and laryngeal cancer risks among men.

*PYs: Person-years of observation
**Those in “unknown” category were excluded when calculating P for trend. Former smokers were also excluded when calculating P for trend for cigarettes smoked a day.
Relative risk was obtained from the following model: H = Hs exp (BiXi), where background hazard, Hs, was stratified by attained age (5 year category), income and education. Xis are categorical variables for cigarette smoking/tobacco chewing.