Howick, Jeremy
(2008)
Philosophical essentials in evidence-based medicine: Evaluating the epistemological role of double blinding and placebo controls.
PhD thesis, London School of Economics and Political Science.
Abstract
The Evidence-Based Medicine (EBM) movement endorses a hierarchy of evidence that places randomized controlled trials at the top. More specifically, double-blind, placebo- controlled trials are often considered to be the 'best of the best'. This view leads to the paradox that treatments that seem to be most strongly supported by evidence, ranging from tracheotomies to rabies vaccines, have never been tested in randomized trials of any description and are hence supported by (allegedly) sub-optimal evidence. Moreover many of these treatments do not seem supportable by best evidence - how, for example do we keep the surgeons who perform tracheotomies 'blind'. After a brief introduction (chapter 1), and review of the literature (chapter 2), I argue that criticisms of the EBM hierarchy can be launched from the simple basis that best evidence rules out the most plausible rival hypothesis (chapter 3). To examine the relative evidential weight of placebo controlled trials compared to 'active' controlled trials (in which the control treatment is an existing established treatment) requires a good deal of conceptual work. I defend a modified version of Grunbaum's (1981/1986) definition of placebos (chapter 4), then provide constraints on what can count as a 'legitimate' placebo control (chapter 5). Next, I explain why double-blinding does not always rule out additional rival hypotheses. I then argue that the arguments for the superiority of placebo controls are flawed. The 'assay sensitivity' argument is limited in scope and based on a misconception about the nature of placebo controls (chapter 7), while the claim that only placebo controlled trials measure the absolute effect size relies on the questionable assumption that placebo and non-placebo effects add rather than interact (chapter 8). I conclude that the evidence hierarchy endorsed by EBM does not stand on solid foundations.
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