HIV
	Probability version	Natural frequency version
Medical situation	Imagine being a doctor at an AIDS counseling center. In addition to individual counseling sessions, HIV tests are also carried out in this AIDS counseling center. For this purpose, a blood sample is taken from the client and an HIV test is carried out
	They are currently counseling a low-risk client who has received a positive HIV test result. This client wants to know what this means for him
	For your answer, you will only have the following information available, based on a sample of low risk individuals who have all been HIV tested:
Presentation of information	Text only	Text only
	Tree diagram only	Tree diagram only
Text	The probability of a low-risk person being HIV-infected is 0.01%	100 out of every 1,000,000 low-risk individuals are HIV-infected
	The probability that a person will receive a positive HIV test result when infected with HIV is 99.7%	100 out of 100 people who are infected with HIV receive a positive result in the HIV test
	The likelihood of a person being mistakenly positive for the HIV test while not HIV-infected is 0.0004%	4 out of 999,900 people who are not infected with HIV mistakenly receive a positive result from the ELISA test
Tree diagram	Probability tree (in the version with a tree diagram)	Frequency tree (in the version with a tree diagram)
Question	What is the probability that this person is actually HIV-infected?	How many of those positively tested for HIV are actually HIV-infected?
	Answer: _______	Answer: ____ out of _____