CAUSAL INFERENCE IN THE EMPIRICAL SCIENCES. Judea Pearl University of California Los Angeles (

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1 CAUSAL INFERENCE IN THE EMPIRICAL SCIENCES Judea Pearl University of California Los Angeles (

2 OUTLINE Inference: Statistical vs. Causal distinctions and mental barriers Formal semantics for counterfactuals: definition, axioms, graphical representations Inference to three types of claims: 1. Effect of potential interventions 2. Attribution (Causes of Effects) 3. Direct and indirect effects

3 TRADITIONAL STATISTICAL INFERENCE PARADIGM Data P Joint Distribution Q(P) (Aspects of P) Inference e.g., Infer whether customers who bought product A would also buy product B. Q = P(B A)

4 FROM STATISTICAL TO CAUSAL ANALYSIS: 1. THE DIFFERENCES Probability and statistics deal with static relations Data P Joint Distribution change Inference P Joint Distribution Q(P ) (Aspects of P ) What happens when P changes? e.g., Infer whether customers who bought product A would still buy A if we were to double the price.

5 FROM STATISTICAL TO CAUSAL ANALYSIS: 1. THE DIFFERENCES What remains invariant when P changes say, to satisfy P (price=2)=1 Data P Joint Distribution change Inference Note: P (v) P (v price = 2) P Joint Distribution Q(P ) (Aspects of P ) P does not tell us how it ought to change e.g. Curing symptoms vs. curing diseases e.g. Analogy: mechanical deformation

6 FROM STATISTICAL TO CAUSAL ANALYSIS: 1. THE DIFFERENCES (CONT) 1. Causal and statistical concepts do not mix. CAUSAL Spurious correlation Randomization Confounding / Effect Instrument Holding constant Explanatory variables 2. STATISTICAL Regression Association / Independence Controlling for / Conditioning Odd and risk ratios Collapsibility Propensity score 3. 4.

7 FROM STATISTICAL TO CAUSAL ANALYSIS: 2. MENTAL BARRIERS 1. Causal and statistical concepts do not mix. CAUSAL STATISTICAL Spurious correlation Regression Randomization Association / Independence Confounding / Effect Controlling for / Conditioning Instrument Odd and risk ratios Holding constant Collapsibility Explanatory variables Propensity score 2. No causes in no causes out (Cartwright, 1989) statistical assumptions + data causal assumptions causal conclusions 3. Causal assumptions cannot be expressed in the mathematical language of standard statistics. 4. }

8 FROM STATISTICAL TO CAUSAL ANALYSIS: 2. MENTAL BARRIERS 1. Causal and statistical concepts do not mix. CAUSAL STATISTICAL Spurious correlation Regression Randomization Association / Independence Confounding / Effect Controlling for / Conditioning Instrument Odd and risk ratios Holding constant Collapsibility Explanatory variables Propensity score 2. No causes in no causes out (Cartwright, 1989) statistical assumptions + data causal assumptions causal conclusions 3. Causal assumptions cannot be expressed in the mathematical language of standard statistics. 4. Non-standard mathematics: a) Structural equation models (Wright, 1920; Simon, 1960) b) Counterfactuals (Neyman-Rubin (Y x ), Lewis (x Y)) }

9 WHY CAUSALITY NEEDS SPECIAL MATHEMATICS Scientific Equations (e.g., Hooke s Law) are non-algebraic e.g., Pricing Policy: Double the competitor s price Correct notation: Y := = 2X X = 1 Process information X = 1 Y = 2 The solution Had X been 3, Y would be 6. If we raise X to 3, Y would be 6. Must wipe out X = 1.

10 WHY CAUSALITY NEEDS SPECIAL MATHEMATICS Scientific Equations (e.g., Hooke s Law) are non-algebraic e.g., Pricing Policy: Double the competitor s price Correct notation: (or) Y 2X X = 1 Process information Had X been 3, Y would be 6. If we raise X to 3, Y would be 6. Must wipe out X = 1. X = 1 Y = 2 The solution

11 THE STRUCTURAL MODEL PARADIGM Data Joint Distribution Data Generating Model Q(M) (Aspects of M) M Inference M Invariant strategy (mechanism, recipe, law, protocol) by which Nature assigns values to variables in the analysis.

12 FAMILIAR CAUSAL MODEL ORACLE FOR MANIPILATION X Y Z INPUT OUTPUT

13 STRUCTURAL CAUSAL MODELS Definition: A structural causal model is a 4-tuple V,U, F, P(u), where V = {V 1,...,V n } are observable variables U = {U 1,...,U m } are background variables F = {f 1,..., f n } are functions determining V, v i = f i (v, u) P(u) is a distribution over U P(u) and F induce a distribution P(v) over observable variables

14 STRUCTURAL MODELS AND CAUSAL DIAGRAMS The arguments of the functions v i = f i (v,u) define a graph v i = f i (pa i,u i ) PA i V \ V i U i U Example: Price Quantity equations in economics U 1 I W U 2 q p = = b b 1 2 p q + + d d 1 i 2 + w u + 1 u 2 Q P PA Q

15 STRUCTURAL MODELS AND INTERVENTION Let X be a set of variables in V. The action do(x) sets X to constants x regardless of the factors which previously determined X. do(x) replaces all functions f i determining X with the constant functions X=x, to create a mutilated model M x q p = = b b 1 2 p q + + d d 1 i 2 + w u + 1 u 2 U 1 I W U 2 Q P

16 STRUCTURAL MODELS AND INTERVENTION Let X be a set of variables in V. The action do(x) sets X to constants x regardless of the factors which previously determined X. do(x) replaces all functions f i determining X with the constant functions X=x, to create a mutilated model M x M p q p p = = = b1 p b2q p0 + + d1i + u1 d2w + u2 U 1 I W U 2 Q P P = p 0

17 CAUSAL MODELS AND COUNTERFACTUALS Definition: The sentence: Y would be y (in situation u), had X been x, denoted Y x (u) = y, means: The solution for Y in a mutilated model M x, (i.e., the equations for X replaced by X = x) with input U=u, is equal to y. The Fundamental Equation of Counterfactuals: Yx ( u) = YM ( u) x

18 ), ( ), ' ( ) ( ) ( ) ( ' ) '( : ' ) ( : y x u P y x y Y PN u P y Y P y P y u u Yx x y u u Yx x = = = = = Δ = = In particular: do(x) CAUSAL MODELS AND COUNTERFACTUALS Definition: The sentence: Y would be y (in situation u), had X been x, denoted Y x (u) = y, means: The solution for Y in a mutilated model M x, (i.e., the equations for X replaced by X = x) with input U=u, is equal to y. ) ( ) ( u Y u Y M x x = The Fundamental Equation of Counterfactuals: ) ( ), ( ) (, ) ( : u P z Z y Y P z u Z y u u Y w x w x = = = = = Joint probabilities of counterfactuals:

19 AXIOMS OF CAUSAL COUNTERFACTUALS Y x ( u) = y : Y would be y, had X been x (in state U = u) 1. Definiteness x X s. t. X y ( u) = 2. Uniqueness 3. Effectiveness X xw ( u) = 4. Composition ( X y ( u) = x) & ( X y( u) = x') x = x' x W x ( u) = w Yxw( u) = Yx ( u) 5. Reversibility ( Yxw ( u) = y & ( Wxy( u) = w) Yx( u) = x y

20 INFERRING THE EFFECT OF INTERVENTIONS The problem: To predict the impact of a proposed intervention using data obtained prior to the intervention. The solution (conditional): Causal Assumptions + Data Policy Claims 1. Mathematical tools for communicating causal assumptions formally and transparently. 2. Deciding (mathematically) whether the assumptions communicated are sufficient for obtaining consistent estimates of the prediction required Deriving Suggesting (if (2) (if (2) is affirmative) is negative) a closed-form set of measurements expression and for experiments the predicted that, impact if performed, would render a consistent estimate feasible.

21 NON-PARAMETRIC STRUCTURAL MODELS Given P(x,y,z), should we ban smoking? Smoking α U1 U 2 X Z Y Tar in Lungs Linear Analysis x = u 1, z = αx + u 2, y = βz + γ u 1 + u 3. β Cancer U 3 f 1 f2 Smoking U1 U 2 X Z Y Tar in Lungs f 3 Cancer U 3 Nonparametric Analysis x = f 1 (u 1 ), z = f 2 (x, u 2 ), y = f 3 (z, u 1, u 3 ). Find: α β Find: P(y do(x))

22 EFFECT OF INTERVENTION AN EXAMPLE Given P(x,y,z), should we ban smoking? U1 U 2 U1 U 3 U2 f 3 U 3 Smoking α X Z Y Linear Analysis x = u 1, z = αx + u 2, y = βz + γ u 1 + u 3. Find: α β Tar in Lungs β Cancer Smoking Find: P(y do(x)) = P(Y=y) in new model f 2 X = x Z Y Tar in Lungs Cancer Nonparametric Analysis x = const. z = f 2 (x, u 2 ), y = f 3 (z, u 1, u 3 ). Δ

23 EFFECT OF INTERVENTION AN EXAMPLE (cont) Given P(x,y,z), should we ban smoking? U (unobserved) U (unobserved) Smoking X Z Y Tar in Lungs Cancer Smoking X = x Z Y Tar in Lungs Cancer

24 EFFECT OF INTERVENTION AN EXAMPLE (cont) Given P(x,y,z), should we ban smoking? U (unobserved) U (unobserved) Smoking X Z Y Tar in Lungs Cancer Smoking X = x Z Y Tar in Lungs Cancer Pre-intervention Post-intervention P ( x, y, z) = P( u) P( x u) P( z x) P( y z, u) P ( y, z do( x)) = P( u) P( z x) P( y z, u) u u

25 EFFECT OF INTERVENTION AN EXAMPLE (cont) Given P(x,y,z), should we ban smoking? U (unobserved) U (unobserved) Smoking X Z Y Tar in Lungs Cancer Smoking X = x Z Y Tar in Lungs Cancer Pre-intervention Post-intervention P ( x, y, z) = P( u) P( x u) P( z x) P( y z, u) P ( y, z do( x)) = P( u) P( z x) P( y z, u) u To compute P(y,z do(x)), we must eliminate u. (graphical problem). u

26 ELIMINATING CONFOUNDING BIAS A GRAPHICAL CRITERION P(y do(x)) is estimable if there is a set Z of variables such that Z d-separates X from Y in G x. G G x Z 1 Z 2 Z 1 Z Z 2 Z 3 Z 3 Z 4 Z 5 Z 4 Z 5 X Z 6 Y X Z 6 Y Moreover, P(y do(x)) = P(y x,z) P(z) z ( adjusting for Z)

27 RULES OF CAUSAL CALCULUS Rule 1: Ignoring observations P(y do{x}, z, w) = P(y do{x}, w) Rule 2: Action/observation exchange P(y do{x}, do{z}, w) = P(y do{x},z,w) Rule 3: Ignoring actions ( Y Z X,W ) P(y do{x}, do{z}, w) = P(y do{x}, w) if if if (Y Z X,W ) ( Y Z X,W ) G X G X Z G X Z(W)

28 DERIVATION IN CAUSAL CALCULUS Genotype (Unobserved) Smoking Tar Cancer P (c do{s}) =Σ t P (c do{s}, t) P (t do{s}) = Σ t P (c do{s}, do{t}) P (t do{s}) = Σ t P (c do{s}, do{t}) P (t s) = Σ t P (c do{t}) P (t s) = Σ s Σ t P (c do{t}, s ) P (s do{t}) P(t s) = Σ s Σ t P (c t, s ) P (s do{t}) P(t s) = Σ s Σ t P (c t, s ) P (s ) P(t s) Probability Axioms Rule 2 Rule 2 Rule 3 Probability Axioms Rule 2 Rule 3

29 INFERENCE ACROSS DESIGNS Problem: Predict P(y do(x)) from a study in which only Z can be controlled. Solution: Determine if P(y do(x)) can be reduced to a mathematical expression involving only do(z).

30 COMPLETENESS RESULTS ON IDENTIFICATION do-calculus is complete Complete graphical criterion for identifying causal effects (Shpitser and Pearl, 2006). Complete graphical criterion for empirical testability of counterfactuals (Shpitser and Pearl, 2007).

31 THE CAUSAL RENAISSANCE: From Hoover (2004) Lost Causes VOCABULARY IN ECONOMICS From Hoover (2004) Lost Causes

32 THE CAUSAL RENAISSANCE: USEFUL RESULTS 1. Complete formal semantics of counterfactuals 2. Transparent language for expressing assumptions 3. Complete solution to causal-effect identification 4. Legal responsibility (bounds) 5. Imperfect experiments (universal bounds for IV) 6. Integration of data from diverse sources 7. Direct and Indirect effects, 8. Complete criterion for counterfactual testability

33 DETERMINING THE CAUSES OF EFFECTS (The Attribution Problem) Your Honor! My client (Mr. A) died BECAUSE he used that drug.

34 DETERMINING THE CAUSES OF EFFECTS (The Attribution Problem) Your Honor! My client (Mr. A) died BECAUSE he used that drug. Court to decide if it is MORE PROBABLE THAN NOT that A would be alive BUT FOR the drug! PN = P(? A is dead, took the drug) > 0.50

35 THE PROBLEM Semantical Problem: 1. What is the meaning of PN(x,y): Probability that event y would not have occurred if it were not for event x, given that x and y did in fact occur.

36 THE PROBLEM Semantical Problem: 1. What is the meaning of PN(x,y): Probability that event y would not have occurred if it were not for event x, given that x and y did in fact occur. Answer: PN( x, y) = P( Yx ' = y' x, y) Computable from M

37 THE PROBLEM Semantical Problem: 1. What is the meaning of PN(x,y): Probability that event y would not have occurred if it were not for event x, given that x and y did in fact occur. Analytical Problem: 2. Under what condition can PN(x,y) be learned from statistical data, i.e., observational, experimental and combined.

38 TYPICAL THEOREMS (Tian and Pearl, 2000) Bounds given combined nonexperimental and experimental data 0 P( y) P( y max P( x,y) x' ) PN 1 P( y' ) min x' P( x,y) Identifiability under monotonicity (Combined data) PN = P( y x) P( y x' ) P( y x) + P( y x' ) P( y P( x,y) x' ) corrected Excess-Risk-Ratio

39 CAN FREQUENCY DATA DECIDE LEGAL RESPONSIBILITY? Experimental Nonexperimental do(x) do(x ) x x Deaths (y) Survivals (y ) ,000 1,000 1,000 1,000 Nonexperimental data: drug usage predicts longer life Experimental data: drug has negligible effect on survival Plaintiff: Mr. A is special. 1. He actually died 2. He used the drug by choice Court to decide (given both data): Is it more probable than not that A would be alive but for the drug? PN Δ = P( Y ' = y' x, y) > x 0.50

40 SOLUTION TO THE ATTRIBUTION PROBLEM WITH PROBABILITY ONE 1 P(y x x,y) 1 Combined data tell more that each study alone

41 EFFECT DECOMPOSITION (direct vs. indirect effects) 1. Why decompose effects? 2. What is the semantics of direct and indirect effects? 3. What are the policy implications of direct and indirect effects? 4. When can direct and indirect effect be estimated consistently from experimental and nonexperimental data?

42 WHY DECOMPOSE EFFECTS? 1. To understand how Nature works 2. To comply with legal requirements 3. To predict the effects of new type of interventions: Signal routing, rather than variable fixing

43 LEGAL IMPLICATIONS OF DIRECT EFFECT Can data prove an employer guilty of hiring discrimination? (Gender) X Z (Qualifications) What is the direct effect of X on Y? (averaged over z) Y (Hiring) E ( Y do( x1 ), do( z)) E( Y do( x0), do( z)) Adjust for Z? No! No!

44 NATURAL SEMANTICS OF AVERAGE DIRECT EFFECTS Robins and Greenland (1992) Pure X Z z = f (x, u) y = g (x, z, u) Y Average Direct Effect of X on Y: The expected change in Y, when we change X from x 0 to x 1 and, for each u, we keep Z constant at whatever value it attained before the change. E[ Y x Y DE( x0, x1; Y ) 1Z x x 0 0 In linear models, DE = Controlled Direct Effect ]

45 SEMANTICS AND IDENTIFICATION OF NESTED COUNTERFACTUALS Consider the quantity Given M, P(u), Q is well defined Q Δ = ( u)] Given u, Z x * (u) is the solution for Z in M x*, call it z Y ( u) is the solution for Y in M xzx*( u) xz experimental Can Q be estimated from data? nonexperimental Experimental: nest-free expression Nonexperimental: subscript-free expression E u [ Y xzx* ( u)

46 NATURAL SEMANTICS OF INDIRECT EFFECTS X Z z = f (x, u) y = g (x, z, u) Y IE( x0, x1; Y ) Indirect Effect of X on Y: The expected change in Y when we keep X constant, say at x 0, and let Z change to whatever value it would have attained had X changed to x 1. E[ Y x Y 0Zx x 1 0 In linear models, IE = TE - DE ]

47 POLICY IMPLICATIONS OF INDIRECT EFFECTS What is the indirect effect of X on Y? The effect of Gender on Hiring if sex discrimination is eliminated. GENDER X Z QUALIFICATION IGNORE f Y HIRING Blocking a link a new type of intervention

48 RELATIONS BETWEEN TOTAL, DIRECT, AND INDIRECT EFFECTS Theorem 5: The total, direct and indirect effects obey The following equality TE( x, x*; Y ) = DE( x, x*; Y ) IE( x*, x; Y ) In words, the total effect (on Y) associated with the transition from x * to x is equal to the difference between the direct effect associated with this transition and the indirect effect associated with the reverse transition, from x to x *.

49 EXPERIMENTAL IDENTIFICATION OF AVERAGE DIRECT EFFECTS Theorem: If there exists a set W such that Y xz Z x* W for all z and x Then the average direct effect DE ( x x*; Y ) = E( Y, ), x Z E( Y *) x* x Is identifiable from experimental data and is given by [ E( Y w) E( Y w) ] DE( x, x*; Y ) = xz x* z P( Zx* = z w) P( w) w, z

50 Theorem: If there exists a set W such that then, DE( x, x*; Y ) Example: GRAPHICAL CONDITION FOR EXPERIMENTAL IDENTIFICATION OF DIRECT EFFECTS ( Y Z W ) and W ND( X Z) = G XZ [ E( Y w) E( Y w) ] xz x* z P( Zx* = z w) P( w) w, z

51 GENERAL PATH-SPECIFIC EFFECTS (Def.) X x * X W Z W Z M g i i = Nonidentifiable even in Markovian models Y z * = Z x* (u) Form a new model, *, specific to active subgraph g f * ( pa, u; g) f ( pa ( g), pa*( g), u ) Definition: g-specific effect Y i E ( x, x* ; Y ) = TE( x, x* ; Y ) g M i i M * g

52 SUMMARY OF RESULTS 1. Formal semantics of path-specific effects, based on signal blocking, instead of value fixing. 2. Path-analytic techniques extended to nonlinear and nonparametric models. 3. Meaningful (graphical) conditions for estimating direct and indirect effects from experimental and nonexperimental data.

53 CONCLUSIONS Structural-model semantics, enriched with logic and graphs, provides: Complete formal basis for causal and counterfactual reasoning Unifies the graphical, potential-outcome and structural equation approaches Provides friendly and formal solutions to century-old problems and confusions.