Causal inference
Using principled approaches to handle noncompliance and imperfect adherence in causal effect estimation.
A practical, enduring exploration of how researchers can rigorously address noncompliance and imperfect adherence when estimating causal effects, outlining strategies, assumptions, diagnostics, and robust inference across diverse study designs.
Published by
Joseph Lewis
July 22, 2025 - 3 min Read
Noncompliance and imperfect adherence create a persistent challenge for causal inference, muddying the link between treatment assignment and actual exposure. In randomized trials and observational studies alike, participants may ignore the assigned protocol, cross over between groups, or only partially engage with the intervention. This introduces bias that standard intention-to-treat estimates fail to correct. A principled response begins with explicit definitions of adherence and nonadherence, then maps these behaviors into the causal estimand of interest. By clarifying who is treated as actually exposed versus assigned, researchers can target estimands such as the local average treatment effect or principal stratum effects. The process invites a careful balance between interpretability and methodological rigor, along with transparent reporting of deviations.
A core step is to model adherence patterns using well-specified, transparent models. Rather than treating noncompliance as noise, researchers quantify it as a process with its own determinants. Covariates, time, and context often shape adherence, making it sensible to employ models that capture these dynamics. Techniques range from instrumental variables to structural equation models and latent class approaches, each with its own assumptions. Importantly, the chosen model should align with the substantive question and the study design. When adherence mechanisms are mischaracterized, estimators can become inconsistent or biased. Rigorous specification, sensitivity analyses, and pre-registration of adherence-related hypotheses can help preserve interpretability and credibility.
Align estimands with adherence realities, not idealized assumptions.
Once adherence is defined, researchers can identify estimands that remain meaningful under imperfect adherence. The local average treatment effect, for example, captures the impact on those whose treatment status is influenced by assignment. This focus acknowledges that not all individuals respond uniformly to a given intervention. Another option is principal stratification, which partitions the population by potential adherence under each treatment. Although such estimands can be appealing theoretically, their identification often hinges on untestable assumptions. The ongoing task is to select estimands that reflect real-world behavior while remaining estimable under plausible models. This balance informs both interpretation and policy relevance.
Identification strategies play a central role in disentangling causal effects from adherence-related confounding. In randomized studies, randomization assists but does not automatically solve noncompliance. Methods like two-stage least squares or generalized method of moments leverage instrumental variables to estimate causal effects among compliers. In observational contexts, propensity score techniques, structural nested models, or g-methods may be employed to adjust for adherence pathways. A principled approach also requires validating the instruments’ relevance and exclusion restrictions, and assessing whether covariates sufficiently capture the mechanisms that relate adherence to outcomes. Robustness checks and graphical diagnostics further guard against fragile conclusions.
Transparency and precommitment strengthen the reliability of conclusions.
Beyond identification, estimation must address precision and uncertainty under imperfect adherence. Standard errors can be inflated when adherence varies across subgroups or over time. Bayesian methods offer a natural framework for propagating uncertainty about adherence processes into causal estimates, enabling probabilistic statements about effects under different adherence scenarios. Empirical Bayes and hierarchical models can borrow strength across units, improving stability when adherence is sparse in some strata. Across methods, transparent reporting of priors, assumptions, and convergence diagnostics is essential. Practitioners should present a range of estimates under plausible adherence patterns, highlighting how conclusions shift as adherence assumptions change.
Diagnostics and sensitivity analyses are indispensable for evaluating the resilience of causal conclusions to adherence misspecification. Posterior predictive checks, falsification tests, and placebo remedies can reveal how sensitive results are to specific modeling choices. Sensitivity analyses might explore stronger or weaker assumptions about the relationship between adherence and outcomes, or examine alternative instruments and adjustment sets. When feasible, researchers can collect auxiliary data on adherence determinants, enabling more precise models. The overarching goal is to demonstrate that substantive conclusions persist under a spectrum of reasonable assumptions, rather than relying on a single, potentially fragile specification.
Methodological rigor meets practical relevance in adherence research.
Designing studies with adherence in mind from the outset improves estimability and credibility. This includes planning randomization schemes that encourage engagement, offering supports that reduce noncompliance, and documenting adherence behavior systematically. Pre-specifying the causal estimand, the modeling toolkit, and the sensitivity analyses reduces researcher degrees of freedom. Reporting adherence patterns alongside outcomes helps readers judge the generalizability of results. When adherence is inherently imperfect, the study’s value lies in clarifying how robust the estimated effects are to these deviations. Such practices facilitate replication and foster trust among policymakers and practitioners.
Advanced causal frameworks unify noncompliance handling with broader causal inference goals. Methods like marginal structural models, g-computation, and sequential models adapt to time-varying adherence by weighting or simulating counterfactual pathways. These approaches can accommodate dynamic treatment regimens and evolving adherence, yielding estimates that reflect realistic exposure histories. Implementations require careful attention to model specification, weight stability, and diagnostic checks for positivity violations. Integrating adherence-aware methods with standard robustness checks creates a comprehensive toolkit for deriving credible causal insights in complex settings.
Pragmatic guidance for researchers and practitioners alike.
In experiments where noncompliance is substantial, per-protocol analyses can be misleading if not properly contextualized. A principled alternative leverages the intent-to-treat effect alongside adherence-aware estimates to provide a fuller picture. By presenting both effects with clear caveats, researchers communicate what outcomes would look like under different engagement scenarios. This dual presentation helps decision-makers weigh costs, benefits, and feasibility. The challenge lies in avoiding overinterpretation of per-protocol results, which can exaggerate effects if selective adherence correlates with unmeasured factors. Clear framing and cautious extrapolation are essential.
In observational studies, where randomization is absent, researchers face additional hurdles in ensuring that adherence-related confounding is addressed. Techniques such as inverse probability weighting or targeted maximum likelihood estimation can mitigate bias from measured factors, but unmeasured adherence determinants remain a concern. A principled stance combines multiple strategies, cross-validates with natural experiments when possible, and emphasizes the plausibility of assumptions. Clear documentation of data quality, measurement error, and the limitations of any proxy adherence indicators strengthens credibility and guides future research to close remaining gaps.
Practitioners can enhance the usefulness of adherence-aware causal estimates by aligning study design, data collection, and reporting with real-world decision contexts. Stakeholders benefit from explicit explanations of who is affected by noncompliance, what would happen under different adherence trajectories, and how uncertainty is quantified. Communicating results in accessible terms without oversimplifying complexities helps bridge the gap between method and policy. In education, medicine, and public health, transparent handling of noncompliance supports better resource allocation and more effective interventions, even when perfect adherence is unattainable.
Looking forward, principled handling of noncompliance will continue to evolve with data richness and computational tools. Hybrid designs that integrate experimental and observational elements promise deeper insights into adherence dynamics. As real-world data streams expand, researchers will increasingly model adherence as a dynamic, context-dependent process, using time-varying covariates and flexible algorithms. The enduring objective remains clear: to produce causal estimates that faithfully reflect how individuals engage with interventions in practice, accompanied by honest assessments of uncertainty and a clear path for interpretation and action.
