jmlr jmlr2008 jmlr2008-20 knowledge-graph by maker-knowledge-mining

20 jmlr-2008-Causal Reasoning with Ancestral Graphs    (Special Topic on Causality)


Source: pdf

Author: Jiji Zhang

Abstract: Causal reasoning is primarily concerned with what would happen to a system under external interventions. In particular, we are often interested in predicting the probability distribution of some random variables that would result if some other variables were forced to take certain values. One prominent approach to tackling this problem is based on causal Bayesian networks, using directed acyclic graphs as causal diagrams to relate post-intervention probabilities to pre-intervention probabilities that are estimable from observational data. However, such causal diagrams are seldom fully testable given observational data. In consequence, many causal discovery algorithms based on data-mining can only output an equivalence class of causal diagrams (rather than a single one). This paper is concerned with causal reasoning given an equivalence class of causal diagrams, represented by a (partial) ancestral graph. We present two main results. The first result extends Pearl (1995)’s celebrated do-calculus to the context of ancestral graphs. In the second result, we focus on a key component of Pearl’s calculus—the property of invariance under interventions, and give stronger graphical conditions for this property than those implied by the first result. The second result also improves the earlier, similar results due to Spirtes et al. (1993). Keywords: ancestral graphs, causal Bayesian network, do-calculus, intervention

Reference: text


Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 One prominent approach to tackling this problem is based on causal Bayesian networks, using directed acyclic graphs as causal diagrams to relate post-intervention probabilities to pre-intervention probabilities that are estimable from observational data. [sent-6, score-0.749]

2 In consequence, many causal discovery algorithms based on data-mining can only output an equivalence class of causal diagrams (rather than a single one). [sent-8, score-0.633]

3 This paper is concerned with causal reasoning given an equivalence class of causal diagrams, represented by a (partial) ancestral graph. [sent-9, score-0.785]

4 Keywords: ancestral graphs, causal Bayesian network, do-calculus, intervention 1. [sent-15, score-0.528]

5 A prominent machinery for causal reasoning of this kind is known as causal Bayesian network (Spirtes et al. [sent-20, score-0.584]

6 So even if the causal DAG (with latent variables) is fully known, we may not be able to predict certain intervention effects because we only have data from the marginal distribution over the observed variables instead of the joint distribution over all causally relevant variables. [sent-27, score-0.531]

7 The familiar curse is that very rarely can observational data determine a unique causal structure, and many causal discovery algorithms in the literature output an equivalence class of causal structures based on observational data (Spirtes et al. [sent-32, score-0.983]

8 The first part is what some causal discovery algorithms attempt to achieve, namely, to learn something about the causal structure—usually features shared by all causal structures in an equivalence class—from the pre-intervention distribution of O. [sent-48, score-0.925]

9 First, we extend the do-calculus of Pearl (1995) to the context of ancestral graphs (Section 4), so that the resulting calculus is based on an equivalence class of causal DAGs with latent variables rather than a single one. [sent-58, score-0.657]

10 Our result improves upon the Spirtes-Glymour-Scheines conditions for invariance formulated with respect to the so-called inducing path graphs, whose relationship with ancestral graphs is discussed in Appendix A. [sent-61, score-0.654]

11 In a causal Bayesian network, the DAG G is interpreted causally, as a representation of the causal structure over V. [sent-64, score-0.584]

12 The postulate that the (pre-intervention) joint distribution P factorizes according to the causal DAG G is known as the causal Markov condition. [sent-67, score-0.584]

13 Note that in the case of a null intervention (when X = Ø), the intervention principle implies the factorization of the pre-intervention distribution P according to G , which is just the causal Markov 1439 Z HANG condition. [sent-75, score-0.522]

14 So the intervention principle generalizes the causal Markov condition: it assumes that the post-intervention distribution also satisfies the causal Markov condition with the post-intervention causal graph. [sent-76, score-0.991]

15 Given a set of variables V, and two variables A, B ∈ V, a variable C (not necessarily included in V) is called a common direct cause of A and B relative to V if C has a direct causal influence on A and also a direct causal influence on B relative to V ∪ {C}. [sent-82, score-0.726]

16 Taking both complications into account, the interesting question is this: what causal reasoning is warranted given the causal information learnable by algorithms that do not assume causal sufficiency for the set of observed variables, such as the FCI algorithm presented in Spirtes et al. [sent-90, score-0.876]

17 In Figure 1(a), for example, B is a collider on the path A, B, D , but is a non-collider on the path C, B, D . [sent-128, score-0.568]

18 A collider path is a path on which every vertex except for the endpoints is a collider. [sent-129, score-0.64]

19 For example, in Figure 1(a), the path C, A, B, D is a collider path because both A and B are colliders on the path. [sent-130, score-0.589]

20 An inducing path relative to L is a path on which every vertex not in L (except for the endpoints) is a collider on the path and every collider is an ancestor of an endpoint of the path. [sent-132, score-1.314]

21 For example, any single-edge path is trivially an inducing path relative to any set of vertices (because the definition does not constrain the endpoints of the path). [sent-133, score-0.723]

22 In Figure 1(a), the path C, B, D is an inducing path relative to {B}, but not an inducing path relative to the empty set (because B is not a collider). [sent-134, score-1.145]

23 However, the path C, A, B, D is an inducing path relative to the empty set, because both A and B are colliders on the path, A is an ancestor of D, and B is an ancestor of C. [sent-135, score-0.865]

24 for each pair of variables A, B ∈ O, A and B are adjacent in M G if and only if there is an inducing path between them relative to L in G ; 1442 C AUSAL R EASONING WITH A NCESTRAL G RAPHS 2. [sent-168, score-0.563]

25 So, if G is the causal DAG for O, L , it is fair to call MG the causal MAG for O. [sent-172, score-0.584]

26 Different causal DAGs may correspond to the same causal MAG. [sent-174, score-0.584]

27 A causal MAG thus carries uncertainty about what the true causal DAG is, but also reveals features that must be satisfied by the underlying causal DAG. [sent-176, score-0.876]

28 There is then a natural causal interpretation of the edges in MAGs, derivative from the causal interpretation of DAGs. [sent-177, score-0.633]

29 , there is a latent variable L in the underlying DAG such that there is a directed path from L to A and a directed path from L to B 6 ). [sent-180, score-0.731]

30 1443 Z HANG The causal MAG that corresponds to the causal DAG is depicted in Figure 3(a)—which syntactically happens to be a DAG in this case. [sent-193, score-0.603]

31 Assuming the causal Markov condition and its converse, the causal Faithfulness condition,9 there is a provably correct independence-constraintbased algorithm to learn a PAG from an oracle of conditional independence relations (Spirtes et al. [sent-232, score-0.632]

32 ,Vn = Y , is called a possibly directed path from X to Y 11 if for every 0 < i ≤ n, the edge between Vi−1 and Vi is not into Vi−1 . [sent-240, score-0.515]

33 In terms of d-separation, the causal Markov condition says that d-separation in a causal DAG implies conditional independence in the (pre-intervention) population distribution. [sent-252, score-0.606]

34 The causal Faithfulness condition says that d-connection in a causal DAG implies conditional dependence in the (pre-intervention) population distribution. [sent-253, score-0.584]

35 It is not hard to see that if there is a definite m-connecting path between X and Y given Z in a PAG, then in every MAG represented by the PAG, the corresponding path is an m-connecting path between X and Y given Z. [sent-283, score-0.734]

36 For example, in Figure 4 the path I, S, G is definitely m-connecting given L, and this path is m-connecting given L in every member of the equivalence class. [sent-284, score-0.588]

37 Another analogue of m-connecting path is the following: Definition 5 (Possibly m-connecting path) In a partial mixed graph, a path p between vertices X and Y is possibly m-connecting relative to a (possibly empty) set of vertices Z (X,Y ∈ Z) if / i. [sent-286, score-0.635]

38 In the PAG, the path X,Y, Z,W is a possibly m-connecting path but not a definite m-connecting path relative to {Y, Z}, because Y and Z are neither colliders nor definite non-colliders on the path. [sent-293, score-0.775]

39 So unlike a definite m-connecting path, a mere possibly m-connecting path in a PAG does not necessarily correspond to a m-connecting path (or imply the existence of a m-connecting path) in a representative MAG in the equivalence class. [sent-296, score-0.528]

40 1446 C AUSAL R EASONING WITH A NCESTRAL G RAPHS X W X W Y Z Y Z Figure 5: Difference between possible and definite m-connecting paths: in the PAG on the right, X,Y, Z,W is a possibly m-connecting path relative to {Y, Z} but not a definite mconnecting path relative to {Y, Z}. [sent-304, score-0.598]

41 Definition 8 (Visibility) Given a MAG M , a directed edge A → B in M is visible if there is a vertex C not adjacent to B, such that either there is an edge between C and A that is into A, or there is a collider path between C and A that is into A and every vertex on the path is a parent of B. [sent-343, score-1.282]

42 For any A, B ∈ O, if A ∈ AnG (B), and there is an inducing path relative to L between A and B that is into A in G , then there is a directed edge A → B in M that is invisible. [sent-348, score-0.7]

43 Taking the contrapositive of Lemma 9 gives us the fact that if A → B is visible in a MAG, then in every DAG represented by the MAG, there is no inducing path between A and B relative to the set of latent variables that is also into A. [sent-357, score-0.689]

44 This implies that for every such DAG G, G A —the graph resulting from eliminating edges out of A in G—will not contain any inducing path between A and B relative to the set of latent variables, which means that the MAG that represents G A will not contain any edge between A and B. [sent-358, score-0.791]

45 Obviously A ← LAB → B is an inducing path between A and B relative to the set of latent variables. [sent-365, score-0.532]

46 The definition of visibility still makes sense in PAGs, except that we will call a directed edge in a PAG definitely visible if it satisfies the condition for visibility in Definition 8, in order to emphasize that this edge is visible in all MAGs in the equivalence class. [sent-434, score-0.591]

47 For any A, B ∈ O and C ⊆ O that does not contain A or B, if a path p between A and B is mconnecting given C in MYX , then p, the same sequence of variables, forms a possibly m-connecting path between A and B given C in PYX . [sent-450, score-0.502]

48 For our purpose, what we need is the obvious consequence of the lemma that if there is an m-connecting path in MYX , then there is a possibly m-connecting path in PYX . [sent-461, score-0.526]

49 What they meant is that if the conditions are not satisfied, then the causal structure does not entail the invariance, although there may exist some particular distribution compatible with the causal structure such that P(y|z) is invariant under some particular intervention on X. [sent-530, score-0.793]

50 From now on when we speak of invariance entailed by the causal DAG, we mean that the conditions in Proposition 18 are satisfied—or equivalently, that the invariance follows from an application of rule 2 or rule 3 in the DAG-based do-calculus. [sent-531, score-0.569]

51 For any A, B ∈ O and C ⊆ O that does not contain A or B, if there is a path d-connecting A and B given C in G that is into A, then there is a path m-connecting A and B given C in M that is either into A or contains an invisible edge out of A. [sent-556, score-0.712]

52 For example, given the MAG in Figure 3(a), P(L|G, S) is invariant under interventions on S, because the only m-connecting path between L and S given G is L, S , which contains a visible directed edge out of L, and so the relevant clause in Theorem 24, clause (1), is satisfied. [sent-586, score-0.816]

53 By contrast, P(L|G, S) is not entailed to be invariant under interventions on G given the MAG—in the sense that there exists a causal DAG compatible with the MAG given which P(L|G, S) is not entailed to be invariant under interventions on G—because clause (1) is not satisfied. [sent-587, score-0.845]

54 Furthermore, if there is an m-connecting path in M that is either into A or out of A with an invisible directed edge, then there is a definite m-connecting path in P that does not start with a definitely visible edge out of A. [sent-601, score-0.895]

55 P(L|G, S) is also invariant under interventions on S because the only definitely m-connecting path between L and S given {G} is S → L which contains a definitely visible edge out of S, satisfying the relevant clause—clause (1)—in Theorem 30. [sent-638, score-0.598]

56 We include in Appendix A a description of the inducing path graphs (IPGs) as well as their relationship to ancestral graphs. [sent-662, score-0.566]

57 As shown there, syntactically the class of ancestral graphs is a proper subclass of the class of inducing path graphs. [sent-663, score-0.585]

58 Regarding the case in Figure 5, for example, their theorems do not imply that P(W |Y, Z) is entailed to be invariant under interventions on X, due to the presence of the possibly m-connecting path in the graph (which in this case is also the POIPG). [sent-676, score-0.532]

59 In this paper we have provided some results about causal reasoning under weaker causal assumptions, represented by a maximal ancestral graph or a partial ancestral graph, the latter of which is fully testable with observational data (assuming the causal Faithfulness condition). [sent-682, score-1.227]

60 In particular, we justify an independent syntactic definition of inducing path graphs, which makes it clear that syntactically the class of MAGs is a subclass of inducing path graphs. [sent-708, score-0.841]

61 An inducing path graph (IPG) is a directed mixed graph, defined relative to a DAG, through the following construction: Input: a DAG G over O, L Output: an IPG IG over O 1. [sent-709, score-0.644]

62 for each pair of variables A, B ∈ O, A and B are adjacent in I G if and only if there is an inducing path between them relative to L in G ; 2. [sent-710, score-0.563]

63 for each pair of adjacent vertices A, B in IG , mark the A-end of the edge as an arrowhead if there is an inducing path between A and B that is into A, otherwise mark the A-end of the edge as a tail. [sent-711, score-0.898]

64 Furthermore, IG encodes information about inducing paths in the original graph, which in turn implies features of the original DAG that bear causal significance. [sent-713, score-0.519]

65 , On , O1 , then between any pair of adjacent nodes in the cycle, Oi and Oi+1 (1 ≤ i ≤ n and On+1 = O1 ), there is an inducing path between them in G relative 20. [sent-728, score-0.54]

66 By the construction, there is an inducing path relative to L in G between A and O 1 that is into O1 , and an inducing path relative to L in G between B and On that is into On , and for every 1 ≤ i ≤ i − 1, there is an inducing path relative to L in G between O i and Oi+1 that is into both. [sent-741, score-1.399]

67 By Lemma 32 in Appendix B, it follows that there is an inducing path between A and B relative to L in G , and so A and B should be adjacent in I , a contradiction. [sent-742, score-0.54]

68 So there is no inducing path between A and B relative to L in G , which means that A and B are not adjacent in IG . [sent-774, score-0.54]

69 Furthermore, if the said inducing path between V0 and V1 is into V0 , then s is into V0 , and if the said inducing path between Vn−1 and Vn is into Vn , then s is into Vn . [sent-796, score-0.822]

70 ) Lemma 32 gives a way to argue for the presence of an inducing path between two variables in a DAG, and hence is very useful for demonstrating that two variables are adjacent in the corresponding MAG. [sent-804, score-0.538]

71 Proof of Lemma 9 Proof Since there is an inducing path between A and B relative to L in G , A and B are adjacent in M . [sent-806, score-0.54]

72 To show this, it suffices to show that for any C, if in M there is an edge between C and A that is into A or there is a collider path between C and A that is into A and every vertex on the path is a parent of B, then C is adjacent to B, which means that the condition for visibility cannot be met. [sent-809, score-0.91]

73 Given u, u and the fact that A ∈ AnG (B), we can apply Lemma 32 to conclude that there is an inducing path between C and B relative to L in G , which means C and B are adjacent in M. [sent-814, score-0.54]

74 Case 2: There is a collider path p in M between C and A that is into A and every non-endpoint vertex on the path is a parent of B. [sent-815, score-0.69]

75 It follows that there is an inducing path in G between Vi and Vi+1 relative to L such that the path is into Vi+1 , and is into Vi unless possibly Vi = C. [sent-817, score-0.711]

76 Given these inducing paths and the fact that every variable other than C on p is an ancestor of B, we can apply Lemma 32 to conclude that there is an inducing path between C and B relative to L in G , which means C and B are adjacent in M . [sent-818, score-0.868]

77 Since X ← LXY → Y is an inducing path between X and Y relative to L in G , X and Y are adjacent in MG . [sent-835, score-0.54]

78 The edge between them is not Ok−1 ↔ B, for otherwise there would be an inducing path between X and Y relative to L in G that includes fewer observed variables than p does, which contradicts our choice of p. [sent-868, score-0.621]

79 The edge is not Om ← B on pain of making M non-ancestral; and the edge is not Om ↔ B on pain of creating an inducing path that includes fewer observed variables than p does. [sent-873, score-0.712]

80 So the edge between X and B in M is into B, but then there is an inducing path between X and Y relative to L in G that includes fewer observed variables than p does, which is a contradiction with our choice of p. [sent-878, score-0.621]

81 There is no inducing path between X and Y relative to L in G , and hence X and Y are not adjacent in MG . [sent-880, score-0.54]

82 1467 Z HANG For that purpose, we first establish two facts: (1) every directed edge in M GYX is also in MYX ; and (2) for every bi-directed edge S ↔ T in MGYX , S and T are also adjacent in MYX ; and the edge between S and T is either a bi-directed edge or an invisible directed edge in M YX . [sent-886, score-1.143]

83 Furthermore, because GYX is a subgraph of G , any inducing path between P and Q relative to L in GYX is also present in G , and any directed path from P to Q in the former is also present in the latter. [sent-889, score-0.788]

84 Suppose for the sake of contradiction that S → T is visible in MYX , that there is a vertex R not adjacent to T , such that either R∗→ S is in MYX or there is a collider path c in MYX between R and S that is into S on which every collider is a parent of T . [sent-902, score-0.739]

85 Because R → T is visible, by definition, there is a vertex Q not adjacent to T such that Q∗→ R is in M or there is a collider path in M between Q 1468 C AUSAL R EASONING WITH A NCESTRAL G RAPHS and R that is into R on which every collider is a parent of T . [sent-925, score-0.658]

86 On the other hand, suppose there is a collider path c into R on which every collider is a parent of T . [sent-930, score-0.527]

87 Then if there is a collider P on c such that P ↔ S is in M , we immediately get a collider path between Q and S that is into S on which every collider is a parent of T . [sent-931, score-0.641]

88 Case 2: There is a collider path c in MYX between R and S that is into S on which every collider is a parent of T . [sent-934, score-0.527]

89 Thus the edge between S and T is either a bi-directed edge or an invisible directed edge in MYX . [sent-948, score-0.638]

90 1469 Z HANG Proof of Lemma 16 Proof It is not hard to check that for any two variables P, Q ∈ O, if P and Q are adjacent in M YX , then they are adjacent in PYX (though the converse is not necessarily true, because an edge not definitely visible in P may still be visible in M ). [sent-965, score-0.519]

91 23 Let p be a minimal d-connecting path between A and B relative to C in G that is into A, minimal in the sense that no other d-connecting path between A and B relative to C that is into A is composed of fewer variables than p is. [sent-970, score-0.573]

92 For each 0 ≤ n ≤ m − 1, if T[n] is in O, then S0 [n] = T[n]; otherwise T[n] is a (latent) collider on p, which, by the fact that p is d-connecting given C, implies that there is a directed path from T[n] to a member of C. [sent-975, score-0.506]

93 Proof of Lemma 23 Proof Note that because A is not an ancestor of any member of C, if there is a path out of A dconnecting A and B given C in G , the path must be a directed path from A to B. [sent-1011, score-0.925]

94 The sub-sequence of that path consisting of observed variables then constitutes a directed path from A to B in M , which is of course out of A and also m-connecting given C in M . [sent-1013, score-0.579]

95 All we need to show is that if the edge between A and D is not a definitely visible edge A → D in P , then there exists a MAG represented by P in which the edge between A and D is not a visible edge out of A. [sent-1017, score-0.749]

96 Obviously if the edge in P is not A → D, there exists a MAG represented in P in which the edge is not A → D, which follows from the fact that P , by definition, displays all edge marks that are shared by all MAGs in the equivalence class. [sent-1018, score-0.535]

97 Then there exists in M a vertex E not adjacent to D such that either E∗→ A or there is a collider path between E and A that is into A and every collider on the path is a parent of D. [sent-1028, score-0.885]

98 Proof of Lemma 28 Proof Note that since A does not have a descendant in C, an m-connecting path out of A given C in M has to be a directed path from A to B such that every vertex on the path is not in C. [sent-1056, score-0.874]

99 Then a shortest such path has to be uncovered,26 and so will correspond to a definite m-connecting path between A and B given C in P (on which every vertex is a definite non-collider). [sent-1057, score-0.526]

100 A triple of vertices X,Y, Z in a graph is called an unshielded triple if there is an edge between X and Y , an edge between Y and Z, but no edge between X and Z. [sent-1063, score-0.522]


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