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Losing sleep over denominators, Part I: An introduction to the problem of Plioplys 1998

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.

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Radiation and astronaut mortality, Part 2: Gathering the measurements

In order to estimate radiation doses in space, I had to make some basic assumptions. First, I assumed that radiation dose levels would vary by some measurable factors: altitude, orbital inclination, and the type of spacecraft. If this were true, then the dose received would depend on how much time was spent at any given location defined by these parameters. While these assumptions do not account for all the factors according to which radiation dose rates might vary (e.g., solar particle events and variation in the solar cycle may have an impact), they are a reasonable starting point.

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