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Backdoor adjustment identifies an interventional law
Statement
Assume AC and a finite Markovian SCM on a finite DAG . Let and be distinct observed vertices, with a countable-valued treatment and an observed standard-Borel outcome. Let be an observed finite vector of standard-Borel vertices whose vertex set is disjoint from and contains no descendant of . Fix an atom with . Suppose (i) d-separates and in and (ii) a regular conditional propensity is strictly positive -almost surely. Then for every Borel set in the state space of , The right side depends only on the observed law. Thus the law of is an identifiable estimand on any statistical model consisting of such SCMs with the same observed variables. If is real and is integrable, applying the identity to bounded truncations and taking limits gives For a continuous treatment, the corresponding regular conditional formula is identified only for -almost every ; this theorem makes no pointwise claim at a null treatment value.
Facts & Assumptions
Given: The SCM, observed variables, atom , backdoor separation, and positivity stated above.
The graph criterion implies in the Markovian SCM. The backdoor criterion implies conditional exchangeability
Under AC, standard-Borel variables have regular conditional laws and joint laws disintegrate in either declared coordinate direction. Existence of regular conditional distributions for standard borel targets, Disintegration of a joint law on standard borel spaces
On the structural response agrees with the factual outcome: . Recursive structural causal models on finite DAGs
An estimand is identifiable when it is constant over parameter values with the same observational law. Parametric statistical models, Observational equivalence, Estimands on a statistical model, Identifiability of an estimand, Identifiability is constancy on observational classes
Proof
Given: The hypotheses in the statement.
By [F1], and are conditionally independent given . By [F2] take regular conditional laws of given and given . The conditional-independence identity implies that, for -almost every , the latter law equals the former law at . Since is an atom with for -almost every , the measure is equivalent to : its density with respect to is , which is positive and integrates to one. Hence the conditional-law equality at holds for -almost every .
On the event , the factual structural recursion and the recursion with the equation replaced by agree at . Induction down the fixed topological order shows that they agree at every descendant, in particular . Therefore the conditional law of given equals that of given for -almost every . Combining with step 1.1 identifies the conditional law of given with the observed conditional law in the formula.
Disintegrating the joint law of by [F2] gives . Substitute the equality from step 2.1 to obtain the displayed formula. If two SCM parameter values induce the same observed law, the integrand kernel and agree outside their common null sets; positivity makes those sets null for the target integral. Hence the right side, and so the estimand, is constant on each observational-equivalence class by [F4]. The real-valued expectation formula follows by applying the distribution identity to bounded truncations of and using integrability to pass to the limit.
Depends on
- Recursive structural causal models on finite DAGs
- Potential outcomes and consistency
- The backdoor criterion implies conditional exchangeability
- Measure kernel and probability kernel
- Regular conditional distribution
- Existence of regular conditional distributions for standard borel targets
- Disintegration of a joint law on standard borel spaces
- The Axiom of Choice
- Parametric statistical models
- Observational equivalence
- Estimands on a statistical model
- Identifiability of an estimand
- Identifiability is constancy on observational classes
Used by
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Sources
- Judea Pearl, Causal Diagrams for Empirical Research, §3-1, pp. 674–675 (standard reference, not scraped)
- Miguel A. Hernán and James M. Robins, Causal Inference: What If, §§7.2–7.3, pp. 87–89 (standard reference, not scraped)