USMLE Biostats 4: 2x2 Table, Odds Ratio, Relative risk, NNT, NNH and more!

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Welcome to LY Med, where I go over everything you need to know for the USMLE STEP 1, with new videos every day.

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This video will introduce 2x2 tables. This table simplifies data and can be used in many ways to help you with your research studies. Recall some research studies include experimental and observational studies (which includes case control, cohort, and case sectional). Let's discuss the case control, in particular the odds ratio. Recall in this research type, we take a group of people WITH the disease and look retrospectively for risk factors. The formula to calculate this is (ad) divided by (bc). This is the formula for the odds ratio. Now how can we interpret the odds ratio? If OR is greater than 1, there is an increase chance of disease due to the risk factors. If OR = 1, then there is no link between the disease and the risk factor. If OR is less than 1, then there is an inverse and decreased risk.
Let's discuss cohort studies. This is when you take a group of people with RISK factors and follow them prospectively and see if they develop the disease. We like to look for relative risk in these studies.
You may say that there are multiple risk factors that lead to a disease, how can we only look at the risk factor in question? You will have to exclude any outliers, and that is called attributable risk (AR). We just subtract our risk in exposed and risk in unexposed. Now to calculate this even further is the number needed to harm (NNH), which is 1 divided by the attributable risk.

Our last topic will be on a 2x2 table that looks at beneficial treatment of exposure. In the video, we will calculate the relative risk (RR), as well as the relative risk reduction (1-RR). We will also calculate the absolute risk reduction and finally we will look at the number needed to treat (NNT) and that is 1 divided by the ARR.