1. Random Thoughts 2012 (COMP 066) Jan-Michael Frahm Jared Heinly

2. Pregnancy Test Accuracy Home pregnancy tests are 99-99.9% accurate. What is the chance you are pregnant if you have a positive test? 2

3. Pregnancy rate by age group 3

4. Pregnancy test by Age Group that means for 18-19 year olds out of 1000 women we have:  114 that are pregnant in the population  886 that are not pregnant 99% accuracy means 4

5. 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 0.7-35.44 have a positive test 5

6. 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 112.86-64.86 have a positive test 6

7. Probability of being Pregnant looking at all positive results  pregnant with positive test 112.86-64.86  not pregnant with positive test 0.7-35.44 probability of being pregnant is anywhere between 7

8. Tests Accuracy Accuracy of a test is proportion of true results 8

9. 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

10. HIV test for 1000000 people  1000 true positive  999000 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

11. HIV test prevalence in high risk groups can be as high as 10% then positive test result is 11

12. Bayes Rule Bayes rule 12

13. Bayes Rule for HIV Test =99.8% 13

14. 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 2008. Use the Bayes rule to compute the probability. Read in the Moldinov book chapter 6. 14