That depends on what you meant by writing the conditional probability.
Bayes rule says that the probability of testing positive when one has the
disease is calculated as follows:

     Pr(T+ | D+)=(Pr(D+ | T+)*Pr(T+))/Pr(D+)

is that what you mean?


Kyle H. Ambert
Department of Behavioral Neuroscience
Oregon Health & Science University



On 8/8/07, sigalit mangut-leiba <smangut@gmail.com> wrote:
>
> hello,
> i want to do a binomial simulation, by taking 200 var. from one group (x)
> and 300 from another (y).
> the prob. for disease=.6 in both groups.
>
> x <- rbinom(200, 1, .6)
>
> y <- rbinom(300, 1, .6)
> if the person is from group x - the probability to find the disease,
> assuming the person is sick, is .95,
> if he is from group Y its .80.
> i want to know the joint probability: p(the person has the disease and
> tested sick)=P(D+,T+).
> my problem is how to write the conditional prob.
> Thanks for your help, also reference on this subject (binomial simulation)
> would be great.
> Sigalit.
>
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