Published and Bad Model


I complained about two glucose modeling papers and an accompanying editorial in the December issue of Clinical Chemistry. My Letter to the editor about one on the papers has been accepted in Clinical Chemistry.

Although not in the Letter, here’s another example of why modeling glucose meter error using average bias plus multiples of the standard deviation (e.g., sampling from a Gaussian distribution) can be misleading. Say truth is 50 mg/dL, which is hypoglycemic and the meter reads 200mg/dL, which is hyperglycemic. This would be a serious error because the provider (or patient if self-monitoring) would administer insulin, when in fact sugar is needed.

But in terms of modeling, say the bias is zero and the glucose CV is 5%. This means the sd at 50 is 2.5 mg/dL. Now to get a value of 200 due to imprecision requires 80 standard deviations! Using a spreadsheet, I can’t get this probability – however, for 30 standard deviations, the probability has 200 zeros to the right of the decimal point followed by a one. In other words not going to happen.

But such errors do occur – albeit rarely – but much more frequently than an 80 standard deviation error.


One Response to Published and Bad Model

  1. […] this strategy will not work to prevent errors in the D zone of an error grid. I mention in my last post that with a bias of zero and a CV of 5%, one could get a D zone error if the observation is 80 […]

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