Posted 10 June 2010 - 04:27 AM
I'm working through the exercise for chapter 6. The initial scenario on page 170 indicates that a positive test was returned for lizard flu. The question posed is what are the probability you have lizard flu given you tested positive. The first piece of data is that there is a 90% probability of lizard flu given a positive test and 9% given a negative test. Page 172 suggests that given a positive test, the probability of having lizard flu is less than 75%. After stepping through the methodology, adding new information about lizard flu (1% of the population has the lizard flu) there is the lizard flu diagnostic test that provides for "the probability that you have it, given that you tested negative" to calculate a 9% chance of having the lizard flu. Then on page 186 to change your base rate it says "9% of people who tested positively have lizard flu."
Can I get a little help understanding how the initial question of what is the actual probability you have lizard flu given you tested positive (p. 170) turned to the probability you have lizard flu given you tested negative (p. 180) concludes that 9% of people who tested positively have lizard flu (186)?