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other developmental disabilities

Developmental disabilities other than cerebral palsy or Down syndrome

Losing sleep over denominators, Part III: Explaining the error and the bias in Plioplys 1998

As we saw in Part II of this three part series, there is something wrong with Figure 4 in Plioplys et al. 1998.1 As a matter of fact, there is something wrong with all of the figures in the study, though it is a bit more difficult in some cases to confirm this. If the reader should like to work out the details for another, Figure 2 is a fairly easy place to start. In Figure 4, as we have seen, data that should apparently run out by age 11 years continues all the way to age 34 years. Did the authors create these curves out of whole cloth?

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Losing sleep over denominators, Part II: The survival curves in Plioplys et al. 1998 cannot be right

We now turn our attention to 1998 study by Plioplys et al.,1 and in particular to one of the many figures in the study: Figure 4. We choose to focus on this figure because the number of individuals involved in the analyses is small enough to make it very easy to see that something is wrong.

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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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