Graphs and Conditional Independence

Transcription

1 CIMPA Summerschool, Hammamet 2011, Tunisia September 5, 2011

2 A directed graphical model Directed graphical model (Bayesian network) showing relations between risk factors, diseases, and symptoms.

3 A pedigree Graphical model for a pedigree from study of Werner s syndrome. Each node is itself a graphical model.

4 A large pedigree p1379 p1380 p1381 p1382 p1383 p1384 p1385 p1386 p1387 p1129 p1128 p1133 p1134 p1135 p1136 p1137 p1536 p1535 p1131 p1408 p1407 p1406 p1405 p1404 p1126 p1396 p1395 p1394 p1393 p1392 p1391p1390 p1389 p1388 p1127 p1426 p1425 p1424 p1423 p1130 p1565p1564 p1530 p1132 p1073 p1457 p1456 p1455 p1454 p1078 p1349 p1079 p1226 p1225 p1224 p1223 p1222p1221 p1220 p1076 p1242 p1241p1240 p1239 p1238 p1237 p1561 p1236 p1575 p1235 p1499 p1234 p1480 p1511 p1534 p1233 p1219 p1074 p1376 p1375 p1374 p1373 p1372 p1371 p1370 p1369 p1368 p1077 p1232 p1231 p1230 p1229 p1228p1227 p1072 p1438p1437 p1436 p1435 p1434 p1439 p1080 p1362 p1361 p1360 p1190 p1449 p1450 p1451 p1452 p1453 p1187 p1427 p1428 p1429 p1430 p1431 p1432 p1433 p1184 p1189 p1188 p1186 p1185 p1075 p927 p928 p929 p331 p544 p543 p542 p328 p1585 p657 p659 p660 p1556 p1555 p1557 p767 p1514 p1568 p765 p875 p876 p877 p766 p661 p662 p745 p842 p1367 p1366 p1365 p1364 p1363 p746 p747 p658 p506 p744 p742 p743 p512 p712 p709 p711 p784 p783 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5 Independence Formal definition Fundamental properties We recall that two random variables X and Y are independent if or, equivalently, if P(X A Y = y) = P(X A) P{(X A) (Y B)} = P(X A)P(Y B). For discrete variables this is equivalent to p ij = p i+ p +j where p ij = P(X = i, Y = j) and p i+ = j p ij etc., whereas for continuous variables the requirement is a factorization of the joint density: f XY (x, y) = f X (x)f Y (y). When X and Y are independent we write X Y.

6 Independence Formal definition Fundamental properties Formal definition Random variables X and Y are conditionally independent given the random variable Z if L(X Y, Z) = L(X Z). We then write X Y Z (or X P Y Z) Intuitively: Knowing Z renders Y irrelevant for predicting X. Factorisation of densities: X Y Z f XYZ (x, y, z)f Z (z) = f XZ (x, z)f YZ (y, z) a, b : f (x, y, z) = a(x, z)b(y, z).

7 Independence Formal definition Fundamental properties For several variables, complex systems of conditional independence can for example be described by undirected graphs. Then a set of variables A is conditionally independent of set B, given the values of a set of variables C if C separates A from B. For example in picture above 1 {4, 7} {2, 3}, {1, 2} 7 {4, 5, 6}.

8 Directed graphical models Independence Formal definition Fundamental properties Directed graphs are also natural models for conditional indpendence: Any node is conditional independent of its non-descendants, given its immediate parents. So, for example, in the above picture we have 5 {1, 4} {2, 3}, 6 {1, 2, 4} {3, 5}.

9 Independence Formal definition Fundamental properties For random variables X, Y, Z, and W it holds (C1) If X Y Z then Y X Z; (C2) If X Y Z and U = g(y ), then X U Z; (C3) If X Y Z and U = g(y ), then X Y (Z, U); (C4) If X Y Z and X W (Y, Z), then X (Y, W ) Z; If density w.r.t. product measure f (x, y, z, w) > 0 also (C5) If X Y (Z, W ) and X Z (Y, W ) then X (Y, Z) W.

10 Proof of (C5): We have Independence Formal definition Fundamental properties X Y (Z, W ) f (x, y, z, w) = a(x, z, w)b(y, z, w). Similarly X Z (Y, W ) f (x, y, z, w) = g(x, y, w)h(y, z, w). If f (x, y, z, w) > 0 for all (x, y, z, w) it thus follows that g(x, y, w) = a(x, z, w)b(y, z, w)/h(y, z, w). The left-hand side does not depend on z. So for fixed z = z 0 : g(x, y, w) = ã(x, w) b(y, w). Insert this into the second expression for f to get f (x, y, z, w) = ã(x, w) b(y, w)h(y, z, w) = a (x, w)b (y, z, w) which shows X (Y, Z) W.

11 Graphoids and semi-graphoids Examples can be seen as encoding abstract irrelevance. With the interpretation: Knowing C, A is irrelevant for learning B, (C1) (C4) translate into: (I1) If, knowing C, learning A is irrelevant for learning B, then B is irrelevant for learning A; (I2) If, knowing C, learning A is irrelevant for learning B, then A is irrelevant for learning any part D of B; (I3) If, knowing C, learning A is irrelevant for learning B, it remains irrelevant having learnt any part D of B; (I4) If, knowing C, learning A is irrelevant for learning B and, having also learnt A, D remains irrelevant for learning B, then both of A and D are irrelevant for learning B. The property analogous to (C5) is slightly more subtle and not generally obvious.

12 Graphoids and semi-graphoids Examples An independence model σ is a ternary relation over subsets of a finite set V. It is graphoid if for all subsets A, B, C, D: (S1) if A σ B C then B σ A C (symmetry); (S2) if A σ (B D) C then A σ B C and A σ D C (decomposition); (S3) if A σ (B D) C then A σ B (C D) (weak union); (S4) if A σ B C and A σ D (B C), then A σ (B D) C (contraction); (S5) if A σ B (C D) and A σ C (B D) then A σ (B C) D (intersection). Semigraphoid if only (S1) (S4) holds. It is compositional if also (S6) if A σ B C and A σ D C then A σ (B D) C (composition).

13 Separation in undirected graphs Graphoids and semi-graphoids Examples Let G = (V, E) be finite and simple undirected graph (no self-loops, no multiple edges). For subsets A, B, S of V, let A G B S denote that S separates A from B in G, i.e. that all paths from A to B intersect S. Fact: The relation G on subsets of V is a compositional graphoid. This fact is the reason for choosing the name graphoid for such independence model.

14 Graphoids and semi-graphoids Examples Systems of random variables For a system V of labeled random variables X v, v V, we use the shorthand A B C X A X B X C, where X A = (X v, v A) denotes the variables with labels in A. The properties (C1) (C4) imply that satisfies the semi-graphoid axioms for such a system, and the graphoid axioms if the joint density of the variables is strictly positive. A regular multivariate Gaussian distribution, defines a compositional graphoid independence model.

15 Graphoids and semi-graphoids Examples Geometric orthogonality Let L, M, and N be linear subspaces of a Hilbert space H and L M N (L N) (M N), where L N = L N.L and M are said to meet orthogonally in N. (O1) If L M N then M L N; (O2) If L M N and U is a linear subspace of L, then U M N; (O3) If L M N and U is a linear subspace of M, then L M (N + U); (O4) If L M N and L R (M + N), then L (M + R) N. Intersection does not hold in general whereas composition (S6) does.

16 Definitions Structural relations among Markov properties G = (V, E) simple undirected graph; An independence model σ satisfies (P) the pairwise Markov property if (L) the local Markov property if (G) the global Markov property if α β α σ β V \ {α, β}; α V : α σ V \ cl(α) bd(α); A G B S A σ B S.

17 Pairwise Markov property Definitions Structural relations among Markov properties Any non-adjacent pair of random variables are conditionally independent given the remaning. For example, 1 σ 5 {2, 3, 4, 6, 7} and 4 σ 6 {1, 2, 3, 5, 7}.

18 Local Markov property Definitions Structural relations among Markov properties Every variable is conditionally independent of the remaining, given its neighbours. For example, 5 σ {1, 4} {2, 3, 6, 7} and 7 σ {1, 2, 3} {4, 5, 6}.

19 Global Markov property Definitions Structural relations among Markov properties To find conditional independence relations, one should look for separating sets, such as {2, 3}, {4, 5, 6}, or {2, 5, 6} For example, it follows that 1 σ 7 {2, 5, 6} and 2 σ 6 {3, 4, 5}.

20 Definitions Structural relations among Markov properties For any semigraphoid it holds that (G) (L) (P) If σ satisfies graphoid axioms it further holds that (P) (G) so that in the graphoid case (G) (L) (P). The latter holds in particular for, when f (x) > 0.

21 Definitions Structural relations among Markov properties (G) (L) (P) (G) implies (L) because bd(α) separates α from V \ cl(α). Assume (L). Then β V \ cl(α) because α β. Thus bd(α) ((V \ cl(α)) \ {β}) = V \ {α, β}, Hence by (L) and weak union (S3) we get that α σ (V \ cl(α)) V \ {α, β}. Decomposition (S2) then gives α σ β V \ {α, β} which is (P).

22 Definitions Structural relations among Markov properties (P) (G) for graphoids: Assume (P) and A G B S. We must show A σ B S. Wlog assume A and B non-empty. Proof is reverse induction on n = S. If n = V 2 then A and B are singletons and (P) yields A σ B S directly. Assume S = n < V 2 and conclusion established for S > n: First assume V = A B S. Then either A or B has at least two elements, say A. If α A then B G (A \ {α}) (S {α}) and also α G B (S A \ {α}) (as G is a semi-graphoid). Thus by the induction hypothesis (A \ {α}) σ B (S {α}) and {α} σ B (S A \ {α}). Now intersection (S5) gives A σ B S.

23 Definitions Structural relations among Markov properties (P) (G) for graphoids, continued For A B S V we choose α V \ (A B S). Then A G B (S {α}) and hence the induction hypothesis yields A σ B (S {α}). Further, either A S separates B from {α} or B S separates A from {α}. Assuming the former gives α σ B A S. Using intersection (S5) we get (A {α}) σ B S and from decomposition (S2) we derive that A σ B S. The latter case is similar.