Posted by **stats-help** on Wednesday, January 23, 2008 at 6:52pm.

7.

In a certain survey, 30% of those questioned are given an incentive to complete the survey. Of those who receive incentives, 80% completed the survey. Of those who do not receive incentives, 25% completed the survey.

a. Find the probability of getting incentives for those who completed the survey. Hint: Use Bayes’ Theorem

b. If one questioned subject is selected at random, find the probability that is completed. Hint: The answer is in part a.

I found the part A answer to be 0.578 by using bayes.

(0.8*0.3)/(0.8*0.3 + 0.25*0.7)

but i have no idea on how to find b. I thought it woudl be .8*.25 because .8 completed the survey with incentives and .25 completed survey so the proabiblity would be 1/(.8*.25).

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