Birdkeeping and Lung Cancer— A Retrospective Observational Study A 1972–1981 health survey in The Hague, Netherlands, discovered an association between keeping pet birds and increased risk of lung cancer. To investigate bird- keeping as a risk factor, researchers conducted a case–control study of patients in 1985 at four hospitals in The Hague (population 450,000). They identified 49 cases of lung cancer among patients who were registered with a general practice, who were age 65 or younger, and who had resided in the city since 1965. They also selected 98 controls from a population of residents having the same general age structure. (Data based on P. A. Holst, D. Kromhout, and R. Brand, “For Debate: Pet Birds as an Independent Risk Factor for Lung Cancer,” British Medical Journal 297 (1988): 13–21.) Data2.csv shows the data gathered on the following variables: FMDSex.1D F;0DM/ AG D Age, in years SS D Socioeconomic status (1 D High, 0 D Low), determined by occupation of the household’s principal wage earner YR D Years of smoking prior to diagnosis or examination CD D Average rate of smoking, in cigarettes per day BK D Indicator of birdkeeping (caged birds in the home for more than 6 consec- utive months from 5 to 14 years before diagnosis (cases) or examination (controls) Age and smoking history are both known to be associated with lung cancer incidence. After age, socioeconomic status, and smoking have been controlled for, is an additional risk associated with birdkeeping?