Practice exercise (made-up values, not clinical guidance). A town has 25 000 inhabitants. At the start of a year, 186 of them have a chronic disease. During the year, 207 more people are newly diagnosed (assume nobody moves away or dies). Find the point prevalence P at the start of the year per 1000 inhabitants. Round to one decimal place.
Practice exercise (made-up values, not clinical guidance). A new test was compared with the reference standard in a study (see the 2 × 2 table). Find the positive predictive value PPV as a percentage. Round to one decimal place.
Practice exercise (made-up values, not clinical guidance). In a cohort study, exposed people contributed 7800 person-years with 281 new cases, and unexposed people 3900 person-years with 70 new cases. Find the incidence rate ratio IRR (exposed versus unexposed). Round to two decimal places.
Practice exercise (made-up values, not clinical guidance). In a randomised trial, 120 patients received a new treatment and 660 received the control treatment. The table shows how many in each group had the outcome (an unwanted event such as a stroke). Find the number needed to treat NNT to prevent one event. Round up to a whole number of patients.
Practice exercise (made-up values, not clinical guidance). A patient has a pre-test probability of 50% for a disease. A test with sensitivity 76% and specificity 87% comes back negative. Find the post-test probability p′ of the disease as a percentage. Round to one decimal place.
Practice exercise (made-up values, not clinical guidance). 10 000 people are screened with a test of sensitivity 87% and specificity 90%. The prevalence of the disease is 16%. How many false-positive results FP are expected?
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