EPH 227d Principles of Screening Please show all your work Question 1: Cervical cancer screening using cytology tests (Pap smears) can detect precancerous lesions and prevent cancer, as well as detect...

Assignment is attached, this is a epidemiology problem



EPH 227d Principles of Screening Please show all your work Question 1: Cervical cancer screening using cytology tests (Pap smears) can detect precancerous lesions and prevent cancer, as well as detect early stage disease and decrease cervical cancer mortality. The prevalence of precancerous lesions in a given population is 4 per 1,000 women. The cytology test has sensitivity 56.4% and specificity 97.3%. The size of your screening population is 100,000 women. [2 point for each part, a through g] a) Set up a 2 x 2 table showing the results. b) How many people in the population truly have precancerous lesions? c) How many people in the population will test positive for precancerous lesions? d) What is the predictive value of a positive cytology test in this population? Suppose instead of the cytology test, you choose to screen your population using an HPV DNA test with sensitivity 97.4% and specificity 94.3%. e) Set up a 2 x 2 table showing the results. f) What is the positive predictive value of the HPV test? g) How does this PPV compare to that of cytology testing (in part d)? Is this difference driven by the difference in the sensitivity or specificity between the two tests? Question 2: Before the introduction of PSA screening in a population, 83% of prostate cancers were diagnosed at a localized stage, with 17% diagnosed at a distant stage. After PSA screening became widespread, 95% of prostate cancers were diagnosed at a localized stage, with only 5% diagnosed at a distant stage. [2 points each] a) Do these numbers indicate a stage shift? Explain your reasoning. b) Describe one other scenario – aside from a stage shift – that would be consistent with these facts. Question 3: Below is a line indicating the natural history of disease for one individual: ABC D E FClinical symptoms would have developed if individual hadn’t been screened Death would have occurred if individual hadn’t been screened Death actually occurred for the screened individual Disease detected by screening, was diagnosed & treated Disease becomes detectable by screening Biological onset of disease Indicate which letters in the diagram are associated with the starting and stopping points for each of the following: [1 point each] a) The detectable pre-clinical phase for this individual b) The lead time for this individual c) Increased survival time that the screening gave this individual 1