outline 1)motivation 2)representing/modeling causal systems 3)estimation and updating 4)model search...

Outline

1) Motivation

2) Representing/Modeling Causal Systems

3) Estimation and Updating

4) Model Search

5) Linear Latent Variable Models

6) Case Study: fMRI

1

Outline

Search I: Causal Bayes Nets

1) Bridge Principles:

Causal Structure Testable Statistical Constraints

2) Equivalence Classes

3) Pattern Search

4) PAG Search

5) Variants

6) Simulation Studies on the Tetrad workbench

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3

Bridge Principles: Acyclic Causal Graph over V Constraints on P(V)

Weak Causal Markov Assumption

V1,V2 causally disconnected V1 _||_ V2

V1 _||_ V2 v1,v2 P(V1=v1 | V2 = v2) = P(V1=v1)

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Weak Causal Markov Assumption

V1,V2 causally disconnected V1 _||_ V2

Causal Markov Axiom

If G is a causal graph, and P a probability distribution over the variables in

G, then in <G,P> satisfy the Markov Axiom iff:

every variable V is independent of its non-effects,

conditional on its immediate causes.

Determinism

(Structural Equations)

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Causal Markov Axiom Acyclicity

d-separation criterion

Independence OracleCausal Graph

Z X Y1

Z _||_ Y1 | X Z _||_ Y2 | X

Z _||_ Y1 | X,Y2 Z _||_ Y2 | X,Y1

Y1 _||_ Y2 | X Y1 _||_ Y2 | X,ZY2


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Faithfulness

Constraints on a probability distribution P generated by a causal structure G hold for all parameterizations of G.

Revenues := b1Rate + b2Economy + eRev

Economy := b3Rate + eEcon

Faithfulness:

b1 ≠ -b3b2

b2 ≠ -b3b1

Tax Rate

Economy

Tax Revenues

b1

b3

b2

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Equivalence Classes

• Independence (d-separation equivalence)• DAGs : Patterns• PAGs : Partial Ancestral Graphs• Intervention Equivalence Classes

• Measurement Model Equivalence Classes• Linear Non-Gaussian Model Equivalence Classes• Etc.

Equivalence:• Independence Equivalence: M1 ╞ (X _||_ Y | Z) M2 ╞ (X _||_ Y | Z)

• Distribution Equivalence: q1 q2 M1(q1) = M2(q2), and vice versa)

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D-separation Equivalence Theorem (Verma and Pearl, 1988) Two acyclic graphs over the same set of variables are

d-separation equivalent iff they have: • the same adjacencies• the same unshielded colliders

d-separation/Independence Equivalence

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Colliders

Y: Collider

Shielded Unshielded

X

Y

Z

X

Y

Z X

Y

Z

Y: Non-Collider X

Y

Z X

Y

ZX

Y

Z

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D-separationX is d-separated from Y by Z in G iffEvery undirected path between X and Y in G is inactive relative to Z

An undirected path is inactive relative to Z iffany node on the path is inactive relative to Z

A node N is inactive relative to Z iffa) N is a non-collider in Z, orb) N is a collider that is not in Z,

and has no descendant in Z

X YZ1

Z2

V

W

Undirected Paths between X , Y:

1) X --> Z1 <-- W --> Y

2) X <-- V --> Y

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D-separationX is d-separated from Y by Z in G iffEvery undirected path between X and Y in G is inactive relative to Z

An undirected path is inactive relative to Z iffany node on the path is inactive relative to Z

A node N is inactive relative to Z iffa) N is a non-collider in Z, orb) N is a collider that is not in Z,

and has no descendant in Z

X YZ1

Z2

V

W

Undirected Paths between X , Y:

1) X --> Z1 <-- W --> Y

2) X <-- V --> Y

X d-sep Y relative to Z = {V} ?

X d-sep Y relative to Z = {V, Z1 } ?

X d-sep Y relative to Z = {W, Z2 } ?

No

Yes

No

X d-sep Y relative to Z = ?

Yes

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D-separation

X3 X2 X1

X3 and X1 d-sep by X2?

Yes: X3 _||_ X1 | X2

X3

T

X2 X1

X3 and X1 d-sep by X2?

No: X3 _||_ X1 | X2

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Statistical Control ≠ Experimental Control

X3

T

X2 X1

X3

T

X2 X1

I

X3 _||_ X1 | X2

X3 _||_ X1 | X2(set)

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Independence Equivalence Classes:Patterns & PAGs

• Patterns (Verma and Pearl, 1990): graphical representation of d-separation equivalence among models with no latent common causes (i.e., causally sufficient models)

• PAGs: (Richardson 1994) graphical representation of a d-separation equivalence class that includes models with latent common causes and sample selection bias that are Markov equivalent over a set of measured variables X

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Patterns

X2 X1

X2 X1

X2 X1

X4 X3

X2 X1

Possible Edges Example

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Patterns: What the Edges Mean

X2 X1

X2 X1 X1 X2 in some members of the equivalence class, and X2 X1 in others.

X1 X2 (X1 is a cause of X2) in every member of the equivalence class.

X2 X1 X1 and X2 are not adjacent in any member of the equivalence class

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Patterns

X2

X4 X3

X1

X2

X4 X3

Represents

Pattern

X1 X2

X4 X3

X1

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Tetrad Demo

1) Load Session: patterns1.tet

2) Change Graph3 minimally to reduce number of equivalent

DAGs maximally

3) Compute the DAGs that are equivalent to your original 3

variable DAG

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Constraint Based Search

Background Knowledge

e.g., X2 prior in time to X3

X3 | X2 X1

Statistical Constraints

Data

Statistical Inference

X2 X3 X1

Equivalence Class of Causal Graphs

X2 X3 X1

X2 X3 X1

Discovery Algorithm

Causal Markov Axiom (D-separation)

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Score Based Search

Background Knowledge

e.g., X2 prior in time to X3

Data

Model Score

X2 X3 X1


X2 X3 X1

X2 X3 X1


X2 X3 X1

X2 X3 X1

X2 X3 X1


X2 X3 X1

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Overview of Search Methods

Constraint Based Searches• TETRAD (PC, FCI)• Very fast – capable of handling 1,000 variables• Pointwise, but not uniformly consistent

Scoring Searches• Scores: BIC, AIC, etc.• Search: Hill Climb, Genetic Alg., Simulated Annealing• Difficult to extend to latent variable models• Meek and Chickering Greedy Equivalence Class (GES)• Very slow – max N ~ 30-40• Pointwise, but not uniformly consistent

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Tetrad Demo

1) Open new session

2) Template: Search from Simulated Data

3) Create Graph, parameterize, instantiate, generate data N=50

4) Choose PC search, execute

5) Attach new search node, choose GES, execute

6) Play (sample size, parameters, alpha value, etc.)

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Tetrad Demo

1) Open new session

2) Load Charity.txt

3) Create Knowledge:

a. Tangibility is exogenous

b. AmountDonate is Last

c. Tangibility direct cause of Imaginability

4) Perform Search

5) Estimate output

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PAGs: Partial Ancestral Graphs

X2

X3

X1

X2

X3

Represents

PAG

X1 X2

X3

X1

X2

X3

T1

X1

X2

X3

X1

etc.

T1

T1 T2

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Z2

X

Z1

Z2

X3

Represents

PAG

Z1 Z2

X3

Z1

etc.

T1

Y

Y Y

Z2

X3

Z1 Z2

X3

Z1

T2

Y Y

T1

T1

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X2 X1

X2 X1

X2 X1

X2 There is a latent commoncause of X1 and X2

No set d-separates X2 and X1

X1 is a cause of X2

X2 is not an ancestor of X1

X1

X2 X1 X1 and X2 are not adjacent

What PAG edges mean.

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1) Adjacency2) Orientation

Constraint-based Search

Constraint-based Search: Adjacency

1. X and Y are adjacent if they are dependent conditional on all subsets that don’t include them

2. X and Y are not adjacent if they are independent conditional on any subset that doesn’t include them

Search: Orientation

Patterns

Y Unshielded

X Y Z

X _||_ Z | YX _||_ Z | Y

Collider Non-Collider

X Y Z X Y Z

X Y Z

X Y Z

X Y Z

Search: Orientation

PAGs

Y Unshielded

X Y Z

X _||_ Z | YX _||_ Z | Y

Collider Non-Collider

X Y Z X Y Z

Search: Orientation

X3

X2

* X1

X1 X3 | X2

1) X1 - X2 adjacent, and into X2. 2) X2 - X3 adjacent 3) X1 - X3 not adjacent

No Yes

X3

X2

* X1 X3

X2

* X1

Test

Test Conditions

Away from Collider

X1

X2

X3 X4

Causal Graph

Independcies

Begin with:

From

X1

X2

X3 X4

X1 X2

X1 X4 {X3}

X2 X4 {X3}

X1

X2

X3 X4

X1

X2

X3 X4

X1

X2

X3 X4

From

From

X1 X2

X1 X4 {X3}

X2 X4 {X3}

Search: Orientation

X4 X3

X2

X1

X4 X3

X2

X1

X4 X3

X2

X1

X4 X3

X2

X1

X4 X3

X2

X1

PAG Pattern

X4 X3

X2

X1

X1 || X2

X1 || X4 | X3

X2 || X4 | X3

After Orientation Phase

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Interesting Cases

X Y Z

L

X

Y

Z2

L1

M1M2

M3

Z1L2

X1

Y2

L1

Y1

X2

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Tetrad Demo

1) Open new session

2) Create graph for M1, M2, M3 on previous slide

3) Search with PC and FCI on each graph, compare results

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Tetrad Demo

1) Open new session

2) Load data: regression_data

3) X is “putative cause”, Y is putative effect,

Z1,Z2 prior to both (potential confounders)

4) Use regression to estimate effect of X on Y

5) Apply FCI search to data

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Variants

1) CPC, CFCI

2) Lingam

LiNGAM

1. Most of the algorithms included in Tetrad (other than KPC) assume causal graphs are to be inferred from conditional independence tests.

2. Usually tests that assume linearity and Gaussianity.

3. LiNGAM uses a different approach.

4. Assumes linearity and non-Gaussianity.

5. Runs Independent Components Analysis (ICA) to estimate the coefficient matrix.

6. Rearranges the coefficient matrix to get a causal order.

7. Prunes weak coefficients by setting them to zero.

ICA

Although complicated, the basic idea is very simple. a11 X1 + ... + a1n Xn = e1

...

an1 X1 + ... + ann Xn = en

Assume e1,...,en are i.i.d.

Try to maximize the non-Gaussianity of w1 X1 + ... + wn Xn = ?

There are n ways to do it up to symmetry! (Cf. Central Limit Theorem, Hyavarinen et al., 2002)

You can use the coefficients for e1, or for e2, or for...

All other linear combinations of e1,...,en are more Gaussian.

ICA

This equation is usually denoted Wx = s But also X = BX + s where B is the coefficient matrix

So Wx = (I – B)x = e

s is the vector of independent components

x is the vector of variables

Just showed that under strong conditions we can estimate W.

So we can estimate B! (But with unknown row order)

Using assumptions of linearity and non-Gaussianity (of all but one variable) alone.

More sophisticated analyses allow errors to be non-i.i.d.

LiNGAM

LiNGAM runs ICA to estimate the coefficient matrix B. The order of the errors is not fixed by ICA, so some rearranging of

the B matrix needs to be done. Rows of the B matrix are swapped so the it is lower triangular. a[i][j] should be non-zero (representing an edge) just in case ij Typically, a cutoff is used to determine if a matrix element is zero. The rearranged matrix corresponds to the idea of a causal order.

LiNGAM

Once you know which nodes are adjacent in the graph and what the causal order is, you can infer a complete DAG.

Review: Use data from a linear non-Gaussian model (all but one variable non-

Gaussian)

Infer a complete DAG (more than a pattern!)

Hands On

1) Attach a Generalized SEM IM.

2) Attach a data set, simulate 1000 points.

3) Attach a Search box and run LiNGAM.

4) Attach another search box to Data and run PC.

5) Compare PC to LiNGAM.

Special Variants of Algorithms

PC Pattern PC Pattern enforces the requirement that the output of the algorithm

will be a pattern. PCD PCD adds corrective code to PC for the case where some variables

stand in deterministic relationships. This results in fewer edges being removed from the graph. For example, if X _||_ Y | Z but Z determines Y, X---Y is not taken out.


CPC The PC algorithm may jump too quickly to the conclusion that a

collider and noncolliders should be oriented, X->Y<-Z, X---Y---Z The CPC algorithm uses a much more conservative test for colliders

and noncolliders, double and triple checking to make sure they should be oriented, against different adjacents to X and to Z.

The result is a graph with fewer but more accurate orientations.

Hands On

1. Simulate data from a “complicated” DAG using a SEM IM.

1. Choose the Search from Simulated Data item from the Templates menu.

2. Make a random 20 node 20 edge DAG.

3. Parameterize as a linear SEM, accepting defaults.

4. Run CPC.

5. Attach another search box to data.

6. Run PC.

7. Layout the PC graph using Fruchterman-Reingold.

8. Copy the layout to the CPC graph.

9. Open PC and CPC simultaneously and note the differences.


1. CFCI

1. Same idea as for CPC but for FCI instead.

2. KPC

1. The PC algorithm typically uses independence tests that assume linearity.

2. The KPC algorithm makes two changes:

1. It uses a non-parametric independence test.

2. It adds some steps to orient edges that are unoriented in the PC pattern.


1. PcLiNGAM

1. If some variables are Gaussian (more than one), others non-Gaussian, this algorithm applies.

2. Runs PC, then orients the unoriented edges (if possible) using non-Gaussianity.

2. LiNG

1. Extends LiNGAM to orient cycles using non-Gaussianity


1. JCPC

1. Uses a Markov blanket style test to add/remove individual edges, using CPC style orientation.

2. Allows individual adjacencies in the graph to be revised from the initial estimate using the PC adjacency search.

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Simulation Studies with Tetrad

outline 1)motivation 2)representing/modeling causal systems 3)estimation and updating 4)model search...

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