Random Thoughts 2012 (COMP 066) Jan-Michael Frahm Jared Heinly
Pregnancy Test Accuracy Home pregnancy tests are % accurate. What is the chance you are pregnant if you have a positive test? 2
Pregnancy rate by age group 3
Pregnancy test by Age Group that means for year olds out of 1000 women we have: 114 that are pregnant in the population 886 that are not pregnant 99% accuracy means 4
False Positive rate of positive tests if your result truly should be negative inverse of false positive is specificity Pregnancy tests have 0.08% -4% false positive rate out of the 886 not pregnant women have a positive test 5
False Negative rate of positive tests if your result truly should be negative inverse of false negative is specificity Pregnancy tests have 1% -44% false positive negative rate (depending on testing day) out of the 114 pregnant women have a positive test 6
Probability of being Pregnant looking at all positive results pregnant with positive test not pregnant with positive test probability of being pregnant is anywhere between 7
Tests Accuracy Accuracy of a test is proportion of true results 8
Positive HIV What are your chances of having HIV after a positive HIV test? false positive rate of 0.2% (2 out of a 1000 negative test positive, 99.8% specificity) false negative rate of 0.1% (1 out of 1000 test negative when they are positive, sensitivity 99.9%) HIV prevalence in non-risk population is 0.1% 9
HIV test for people 1000 true positive true negatives false positive of 0.2% means 1998 true negatives test positive false negative of 0.1% of positive means 999 true positive tests negative chance of positive test meaning you have HIV is 10
HIV test prevalence in high risk groups can be as high as 10% then positive test result is 11
Bayes Rule Bayes rule 12
Bayes Rule for HIV Test =99.8% 13
Assignment Calculate the probability of being pregnant with a positive pregnancy test for a women with age 27 and for a women of age 44 in Use the Bayes rule to compute the probability. Read in the Moldinov book chapter 6. 14