Full disclosure: We consult professionally in the context of personal-injury litigation. One of us (SMD) has served on more than one occasion as an expert witness opposite Dr Audrius Plioplys.

On 19 June 2013, Dr Tom Frieden, Director of the Centers for Disease Control and Prevention (CDC), delivered the Keynote Address at the 46th Annual Society for Epidemiologic Research (SER) Meeting in Boston, Massachusetts. He began with a quip (paraphrasing): an epidemiologist is someone who loses sleep worrying about denominators. His point was well understood by his audience, including yours truly. Mortality rates, survival probabilities, incidence rates, incidence rate ratios, hazard ratios, odds ratios, and standardized mortality ratios are but a few of the commonly calculated epidemiologic measures that involve dividing one number (the numerator) by another (the denominator). Getting these calculations right can be critical, and when gotten wrong, the consequences can be embarrassing, or worse.