Causal diagrams, the placebo effect, and the expectation effect

Area of Science:

• Biomedical Research
• Clinical Psychology
• Medical Terminology

Background:

• The term "placebo" and "placebo effect" are widely used in biomedical research.
• Existing definitions lack clarity and precision, leading to ambiguity in scientific discourse.

Purpose of the Study:

• To formally analyze definitions of placebo and the placebo effect using causal diagrams.
• To propose a clearer conceptual framework for understanding patient responses in clinical research.

Main Methods:

• Application of causal diagrams as a formal research methodology.
• Analysis of multiple definitions of placebo and the placebo effect.

Main Results:

• The term "placebo" is identified as ambiguous and redundant.
• The "placebo effect" is found to obscure the significant role of patient expectation.
• Key aspects of expectation, including causes, effects, and modifiers, warrant deeper investigation.

Conclusions:

• Abandoning the term "placebo effect" will benefit biomedical research.
• Focusing on the "expectation variable" offers a more precise and fruitful research direction.
• Future research should utilize observational cohorts nested within clinical trials to study expectation.