The 2x2 Table: Risk Ratio and Risk Difference Explained Simply

The 2x2 Table: Risk Ratio and Risk Difference Explained Simply is Lesson 6 of 20 in Principles of Epidemiology. Twice the risk, or 12.5 more cases per hundred? Both are true, and they answer different questions. This lesson builds the two-by-two table and teaches the risk ratio, risk difference and rate ratio with one invented factory cohort. This course is taught by Angel Reyes, MPH, MCHES, PMP, PMI-ACP, a doctoral candidate in public health who earned an MPH from a CEPH-accredited program, has planned, managed, and evaluated public health programs, and builds research training for graduate students and practitioners at Subthesis. Part of the 20-lesson course Principles of Epidemiology. Five key facts from this lesson: 1. The course's two-by-two table puts exposure in rows and outcome in columns, with cells a, b, c and d, and you always read across the rows. 2. In the invented cohort, 100 of 400 exposed and 75 of 600 unexposed become ill: risks of 25 and 12.5 percent. 3. The risk ratio is 25 over 12.5, or 2.0, a relative measure; a value of 1 means no association and below 1 means lower risk among the exposed. 4. The risk difference is 12.5 cases per 100 exposed, an absolute measure of the excess burden. 5. A risk ratio of 2.0 can hide a gap of one case per 10,000, which is why a ratio and a difference answer different questions. Free research tools at https://www.subthesis.com, including the Research Question Builder and the Statistical Power Calculator. Companion courses on this channel: Certified in Public Health: The CPH Exam Pathway, and Foundations of Program Planning. Examples are fictional and all figures are invented for teaching. Oakhaven Health District does not exist. This course is independent educational content from Subthesis and is not clinical, diagnostic, screening, or policy guidance.