Statistical Analysis
The expected number of cases of cancer was calculated on the basis of national incidence rates obtained from the Cancer Registry according to sex, age, and calendar period in five-year intervals. Multiplying the number of person-years of observation by the incidence rates yielded the number of cancer cases that would be expected if patients with deep venous thrombosis and pulmonary embolism had the same risk of cancer as the general population. Confidence intervals for the standardized incidence ratio — i.e., the ratio of observed to expected cancers — were computed on the basis of the assumption that the observed number of cases in a specific category follows a Poisson distribution. Exact limits were used when the observed number was less than 10; otherwise, Byar's approximation was used