Empiric Antibiotic Therapy for Sepsis Patients: Monotherapy With β-Lactam or β-Lactam Plus an Aminoglycoside? Answers to the November 2008 Journal Club Questions

The Episcope. The user looks at the output of the device (level K) and sees the “known” risk difference (^kRD) (or any other measure of effect). The known risk difference results from information transmitted, such as light waves through a telescope, from a causal (“etiologic”) risk difference (^aRD) in a target population, through layers of lenses and filters. Each layer is a distinct domain in which certain types of biases operate, potentially further distorting the estimate of RD from its true values (^aRD). It is only by considering the biases introduced at each of the 10 levels that we can determine to what degree ^kRD is an accurate proxy for ^aRD. This figure was produced by Maclure and Schneeweiss for their article “Causation of Bias: The Episcope.” Their original figure was developed for case-control epidemiologic studies. We have modified their figure slightly to include the bolded terms at each level. These terms indicate the kinds of bias that might be introduced at each level in a randomized trial. From Maclure M, Schneeweiss S. Causation of bias: the episcope. Epidemiology. 2001;12:114-122. Used with permission from Lippincott Williams & Wilkins, Baltimore, MD.

The figure on the left represents scattered values that are somewhat distant from each other (low precision) and yet, on average, approach the target's center (high accuracy/low bias). The figure on the right shows several values that cluster closely together (high precision) and yet, on average, remain distant from the target's center (low accuracy/high bias).