2. Fifty-two males between ages of 11 and 30 were operated on for knee injuries using arthro- scopic surgery. The patients were classied by the type of injury: twisted knee, direct blow, or both. The results of the surgery were classied as either excellent (E), good (G) and fair or poor (FP). E G F-P Twisted 21 l l 4 Both 3 2 2 Direct 7 1 1 (i) Estimate 1,2. (ii) Estimate 1'3, the chance of an excellent outcome given a direct blow, and 1'1 the chance of an excellent outcome given a twisted knee