Epidemiology module of practical skills for mbbs student

Part II. Cohort study
In 1951, a prospective cohort study was undertaken to quantify the relation between smoking and lung cancer. The study population comprised all physicians listed in the Medical Register and resident in England and Wales in October 1951. Information about present and past smoking habits was obtained by questionnaire. Deaths from lung cancer were identified from death certificates and other mortality data recorded during ensuing years.
In October 1951 questionnaires were mailed to 59,600 physicians, seeking self-classification under any of 3 smoking groups: [1] current smokers, [2] ex-smokers and [3] non-smokers. Smokers and ex-smokers were asked the amount they smoked, their method of smoking, the age they started to smoke, and, if they had stopped smoking, how long it had been since they last smoked. Usable responses to the questionnaires were received from 34,445 (85%) of the male physicians.
The occurrence of lung cancer in physicians who responded to the questionnaire was documented over the next 10 years (November 1951 to October 1961) from death certificates filed with the Registrar General and from lists of physician deaths provided by the British Medical Association. All certificates indicating that the deceased was a physician were abstracted. For each lung cancer case, medical records were obtained to confirm the diagnosis. The same clinical and pathologic criteria were used as in the earlier case-control study. Of 4,597 deaths in the cohort over the 10 year period, 216 were reported to have had lung cancer, a diagnosis that could be documented in 4, leaving a net total of 212 cases of lung cancer.
Table 4 shows lung cancer deaths by age and amount smoked. Rates are both age-specific and age standardized (ëall agesí category) and are expressed as deaths per 1,000 person-years at risk.