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. 2013 Sep 27:6:821-8.
doi: 10.2147/IJGM.S52209. eCollection 2013.

Causal diagrams, the placebo effect, and the expectation effect

Eyal Shahar  1 , Doron J Shahar
Affiliations

Causal diagrams, the placebo effect, and the expectation effect

Eyal Shahar et al. Int J Gen Med. .

Abstract

Using causal diagrams, a formal research methodology, we analyzed several definitions of placebo and the placebo effect. We conclude that placebo is an ambiguous, redundant term and that the so-called placebo effect conceals far more interesting effects that are attributed to the patient's expectation. Biomedical research will benefit from abandoning the term placebo effect and focusing instead on a deeper understanding of the expectation variable, including its causes, effects, and effect modifiers. This avenue of research should be pursued by observational cohorts that are nested within clinical trials.

Keywords: causal diagrams; effect modification; expectation; placebo.

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Figures

Figure 1
Figure 1
A causal diagram. Notes: TOFFERED denotes the treatment offered; TTAKEN denotes the treatment taken; D denotes the outcome; R, Q, and C are generic variables; r denotes a value of the modifier R; q denotes a value of the modifier Q.
Figure 2
Figure 2
A causal structure, incorporating causes of the offered treatment (ℛ), blinding status (B), and the patient’s expectation (E) of the outcome (D). Notes: TOFFERED denotes the treatment offered; TTAKEN denotes the treatment taken; a lower case letter denotes a value of the modifier (e.g. tOFFERED is a value of TOFFERED).
Figure 3
Figure 3
A proposed definition of placebo (t0). Notes: TOFFERED denotes the treatment offered; TTAKEN denotes the treatment taken; E denotes the patient’s expectation of the outcome D = d; t denotes a value of TTAKEN.
Figure 4
Figure 4
Various definitions of the placebo effect. Notes: TOFFERED denotes the treatment offered; TTAKEN denotes the treatment taken; E denotes the patient’s expectation of the outcome D = d.
Figure 5
Figure 5
The causal structure of a successful double-blinded, randomized trial. Notes: ℛ denotes all causes of the offered treatment B denotes blinding status; Toffered denotes the treatment offered; TTAKEN denotes the treatment taken; E denotes the patient’s expectation of the outcome D = d; a lower case letter denotes a value of the modifier (e.g. toffered is a value of Toffered) A box denotes conditioning (see text).
Figure 6
Figure 6
Three designs for studying the placebo effect. (A) three arms. (B) Concealed treatment. (C) Balanced placebo design.
See this image and copyright information in PMC

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