In a study performed
a few years ago [1], the following problem was proposed to 18 expert
consultants for AIDS patients:

*Helen has tested positive for AIDS. How likely do you think she is really an AIDS patient? What would you advise her?*
The input
data are:

1. The probability of having AIDS, when
one belongs to a population without special risk, is 1 in 10,000.

2. The sensitivity of the AIDS test is
99.9%. In other words, the probability of a false negative is 0.1%.

3. The specificity of the AIDS test is
99.99%. In other words, the probability of a false positive is 0.01%.

The result
of the study was as follows:

·

**The 18 experts agreed that the probability that Helen is an AIDS patient is greater than 90%. Most thought that the probability is greater than 99%. Some even claimed that is greater than 99.9%.**
·

**All experts said that they would advise Helen to inform her family, make everybody test for AIDS, and start taking medication.**