ok, if it is the second case:

given:
* 100 infected people take the test, 95% turn out HIV阳性. and 100 uninfected people take the test, 5% turn out HIV阳性.
* actual infection rate is 0.1%

if 1,000,000 people take the test, 1,000 of them are infected. but the test result will say 50,950 are infected:
1,000,000 * 0.05 + 1,000 * 0.95 = 50,950

so the 小王感染的概率 is 950/50,950 = 2%

所有跟帖： 

• 2%的答案大错特措了！不可能这么低！ -不轻易下结论- ♀ (10 bytes) () 07/14/2004 postreply 18:57:59

• well, according to definition -junjunbos- ♀ (70 bytes) () 07/14/2004 postreply 19:04:51

• 你的论证很有意思，可以称作悖论了。 -不轻易下结论- ♀ (98 bytes) () 07/14/2004 postreply 19:11:15