Of the 99,500 people without cancer, 1% will test positive, yielding 995 false positives out of 1,470 positive tests. In other words, even if you tested positive for the cancer, the probability that you actually have it is only about 32%.
让我们做一个简单的演算。假设医院共做了10万次检测。其中平均有500人会得癌症。由于患病者检测出阳性的概率是95%，那么平均会475个阳性结果。而对于剩下那9.95万健康人来说，由于出现阳性结果的概率是1%，那么就会有995个假阳性出现，因此这10万人中会有475+995=1470个阳性结果。换句话说，即便你的癌症检测结果呈阳性，你真正患有该病的可能性也只有32%。