Oral Contraceptives and Thromboembolism

To the Editor:— In the debate which has arisen over the report by Vessey and Doll 1 , initiated by Drill and Calhoun 2 and followed by the series ofLetters to Editor 3 from both groups, as well as Frederiksen and Seltzer, it appears that all are skirmishing about in a morass of statistical manipulations of epidemiological procedures without closely scrutinizing the sources of the data used in the study itself. Case selection—and exclusions—must be held both basic and primary, preceding all other considerations. Vessey and Doll 1 searched out all cases of "phlebitis, thrombophlebitis, thrombosis, or embolism in any vein except the cerebral, coronary, hepatic, and mesenteric veins; pulmonary embolism or infarction" as diagnosed in females aged 16 through 40 years at 19 general hospitals of a given area in England during the years 1964 to 1966. They found 399 such patients, but then excluded fully 85% (338) of