Cited by 800Advanced Causal Inference TechniquesEvaluation and Performance AssessmentStatistical Methods and Bayesian Inference

The DOI matches its Crossref record; 48 of the 49 references it cites were found in a registry or an index; none of the 26 sources its claims cite is checked for support yet.

How Conditioning on Posttreatment Variables Can Ruin Your Experiment and What to Do about It

Open access
Montgomery, Jacob M., Nyhan, Brendan, Torres, Michelle·2018·American Journal of Political Science

Abstract

In principle, experiments offer a straightforward method for social scientists to accurately estimate causal effects. However, scholars often unwittingly distort treatment effect estimates by conditioning on variables that could be affected by their experimental manipulation. Typical examples include controlling for posttreatment variables in statistical models, eliminating observations based on posttreatment criteria, or subsetting the data based on posttreatment variables. Though these modeling choices are intended to address common problems encountered when conducting experiments, they can bias estimates of causal effects. Moreover, problems associated with conditioning on posttreatment variables remain largely unrecognized in the field, which we show frequently publishes experimental studies using these practices in our discipline's most prestigious journals. We demonstrate the severity of experimental posttreatment bias analytically and document the magnitude of the potential distortions it induces using visualizations and reanalyses of real‐world data. We conclude by providing applied researchers with recommendations for best practice.

Overview

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Registry

Crossref · 10.1111/ajps.12357

Cites

48 of 48 references verified0 citation errors · 0 not checkable

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52 Own interpretation · 30 Own result · 18 Not checked yet · 12 No basis given · 1 Own method

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  • Probably not a substantial claim. 15
  • It may state two separate claims as one. 22
36 of 36
  • The authors state this as an assumption.

    Treatment assignment is independent of potential outcomes conditional on covariates.

    “Assumption (1) : (y [T =1] , y [T =0] ) ⊥ ⊥ T |X, which states that treatment assignment is independent of potential outcomes conditional on covariates.”
  • The authors state this as an assumption.

    All variables are observed without error.

    “For the sake of expositional clarity, and without loss of generality, we assume that all variables are observed without error.”
  • The authors state this as an assumption.

    The main parameters in Model 4 do not vary as a function of T or x.

    “Note that Model 4 also assumes that the main parameters in the model ( , ␣ Y , ␤, , and Y ) do not vary as a function of T or x, which is why we can move these parameters outside of the expectations.”
  • The authors state this as an assumption.

    The influence of unmeasured confounders along the various causal paths will not somehow cancel out.

    “For instance, we assume that the influence of unmeasured confounders along the various causal paths will not somehow cancel out.”
  • The authors state this as an assumption.

    x has no direct effect on y in the true model.

    “If we instead allow x to have a direct effect on y in the true model, the biases we describe below still hold, but the calculations involved are more complex.We make this simplifying assumption so that we can focus our exposition on the posttreatment bias that arises from unblocking the path from u to y.”
  • The authors state this as an assumption.

    β = 0.

    “To simplify exposition, we focus here only on the bias resulting from the imbalance in u induced by controlling for the posttreatment variable x by assuming that ␤ = 0.”
  • The authors state this as an assumption.

    Critical parameters including the treatment effect, the effect of the treatment on the covariate, and the effects of the confounders on outcomes and covariates were constant for each individual.

    “Second, we made the simplifying assumption in our examples above that critical parameters including the treatment effect ( ), the effect of the treatment on the covariate (␥ ), and the effects of the confounders on outcomes and covariates ( Y , X ) were constant for each individual.”
  • The authors state this as an assumption.

    Missingness is not a function of unmeasured confounders.

    “If we are willing to assume that missingness is not a function of unmeasured confounders, we can use familiar methods such as imputation or marginal structural models.”
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Metadata

Authors
Montgomery, J. M., Nyhan, B., & Torres, M.
Affiliations
  • Washington University in St. Louis (US)
  • Dartmouth College (US)
Venue
American Journal of Political Science
Publisher
Wiley
Published
2018-03-30
Volume
62
Issue
3
Pages
760-775
ISSN
  • 0092-5853 (ISSN-L)
  • 1540-5907
OpenAlex
W2794592087
Cited by
800
References
49
Citations by year
2026: 68 · 2025: 128 · 2024: 120 · 2023: 114 · 2022: 105 · 2021: 89 · 2020: 95 · 2019: 48 · 2018: 27
FWCI
53.73
Type
journal article
Language
en
Open access
Locations
American Journal of Political Science (publishedVersion)
Topics
  • Physical Sciences › Mathematics › Statistics and Probability › Advanced Causal Inference Techniques
  • Social Sciences › Public Administration › Evaluation and Performance Assessment
  • Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods and Bayesian Inference
Keywords
causal effect estimation, experimental design, confounding, bias in treatment effect estimates

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