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. 2020 Dec;7(4):203-211.
doi: 10.1007/s40471-020-00245-2. Epub 2020 Sep 23.

Difference-in-Difference in the Time of Cholera: a Gentle Introduction for Epidemiologists

Ellen C Caniglia  1 , Eleanor J Murray  2
Affiliations

Difference-in-Difference in the Time of Cholera: a Gentle Introduction for Epidemiologists

Ellen C Caniglia et al. Curr Epidemiol Rep. 2020 Dec.

Abstract

Purpose of review: The goal of this article is to provide an introduction to the intuition behind the difference-in-difference method for epidemiologists. We focus on the theoretical aspects of this tool, including the types of questions for which difference-in-difference is appropriate, and what assumptions must hold for the results to be causally interpretable.

Recent findings: While currently under-utilized in epidemiologic research, the difference-in-difference method is a useful tool to examine effects of population-level exposures, but relies on strong assumptions.

Summary: We use the famous example of John Snow's investigation of the cause of cholera mortality in London to illustrate the difference-in-difference approach and corresponding assumptions. We conclude by arguing that this method deserves a second-look from epidemiologists interested in asking causal questions about the impact of a population-level exposure change on a population-level outcome for the group that experienced the change.

Keywords: John Snow; causal inference; change scores; difference in difference.

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Conflict of interest statement

Conflict of Interest Ellen C. Caniglia and Eleanor J. Murray declare that they have no conflict of interest

Figures

Figure 1:
Figure 1:
Causal diagram illustrating the effect of moving the water source upstream on the change in cholera death rate from time t=0 to time t=1. Δt denotes the difference Yt=1 – Yt=0.
Figure 2:
Figure 2:
A graphical representation of the components of the difference-in-difference estimator. The solid red line represents the observed cholera death rate (per 100,000 people) over time in the Southwark & Lambeth households; the solid blue line represents the observed cholera death rate (per 100,000 people) over time in the Lambeth households; and the dashed blue line represents the estimated counterfactual cholera death rate (per 100,000 people) over time in the Lambeth households had Lambeth not changed its water source. Adapted from [4].
See this image and copyright information in PMC

References

    1. Snow J On the Mode of Communication of Cholera. John Churchill; 1855. 216 p.

      The first recorded use of the difference-in-difference method.

    1. Snow J Snow on cholera, being a reprint of two papers. New York: New York, The Commonwealth fund; 1936.

      The first recorded use of the difference-in-difference method.

    1. Wing C, Simon K, Bello-Gomez RA. Designing Difference in Difference Studies: Best Practices for Public Health Policy Research. Annu Rev Public Health. 2018. April;39(1):453–69.

      An excellent review article for those interested in learning more about the applied use of difference-in-difference

    1. Difference-in-Difference Estimation | Columbia Public Health [Internet]. [cited 2020 Jul 17]. Available from: https://www.publichealth.columbia.edu/research/population-health-methods/difference-difference-estimation

      A useful web-based tutorial on the difference-in-difference method.

    1. Subramanian SV, Jones K, Kaddour A, Krieger N. Revisiting Robinson: The perils of individualistic and ecologic fallacy. Int J Epidemiol. 2009. April;38(2):342–60. - PMC - PubMed

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