Transcript Learning-Causality

Learning Causality
Some slides are from Judea Pearl’s class lecture
http://bayes.cs.ucla.edu/BOOK-2K/viewgraphs.html
A causal model Example
Rain
Mud
Other causes of mud
• Statement ‘rain causes mud’ implies an
asymmetric relationship: the rain will create
mud, but the mud will not create rain.
• Use ‘→’ when refer such causal relationship;
• There is no arrow between ‘rain’ and ‘other
causes of mud’ means that there is no direct
causal relationship between them;
Directed (causal) Graphs
A
E
C
B
•
•
•
•
•
F
D
A and B are causally independent;
C, D, E, and F are causally dependent on A and B;
A and B are direct causes C;
A and B are indirect causes D, E and F;
If C is prevented from changing with A and B, then A
and B will no longer cause changes in D, E and F.
Conditional Independence
Conditional Independence
Conditional Independence
(Notation)
Causal Structure
Causal Structure (cont’d)
• A Causal Structure serves as a blueprint for
forming a “casual model” – a precise specification
of how each variable is influenced by its parents
in the DAG.
• We assume that Nature is at liberty to impose
arbitrary functional relationships between each
effect and its causes and then to perturb these
relationships by introducing arbitrary disturbance;
• These disturbances reflect “hidden” or
unmeasurable conditions.
Causal Model
Causal Model (Cont’d)
• Once a causal model M is formed, it defines a joint
probability distribution P(M) over the variables in the
system;
• This distribution reflects some features of the causal
structure
– Each variable must be independent of its grandparents, given the
values of its parents
• We may allowed to inspect a select subset OV of
“observed” variables to ask questions about P[o], the
probability distribution over the observations;
• We may recover the topology D of the DAG, from features
of the probability distribution P[o].
Inferred Causation
Latent Structure
Structure Preference
Structure Preference (Cont’d)
• The set of independencies entailed by a causal structure
imposes limits on its power to mimic other structure;
• L1 cannot be preferred to L2 if there is even one
observable dependency that is permitted by L1 and
forbidden by L2;
• L1 is preferred to L2 if L2 has subset of L1’s
independence;
• Thus, test for preference and equivalence can sometimes
be reduced to test dependencies, which can be
determined by topology of the DAGs without concerning
parameters.
Minimality
Consistency
Inferred Causation
Examples
{a,b,c,d} reveal two independencies:
1. a is independent of b;
2. d is independent of {a,b} given c;
Assume further that the data reveals no other
independencies;
a = having a cold;
b = having hay fever;
c = having to sneeze;
d = having to wipe one’s nose.
Example (Cont’d)
{a,b,c,d} reveal two independencies:
1. a is independent of b;
2. d is independent of {a,b} given c;
Arbitrary relations
between a and b
minimal
Not minimal:
fails to impose
conditional
Independence
between d and
{a,b}
Not consistent
with data:
impose marginal
independence
between d and
{a,b}
Stability
The stability condition states that, as we vary the parmeters from  to ,
no indpendence in P can be destroyed.
In other words, if the independency exists, it will always exists.
Stable distribution
• A probability distribution P is a
faithful/stable distribution if there exist a
directed acyclic graph (DAG) D such that
the conditional independence relationship
in P is also shown in the D, and vice
versa.
IC algorithm (Inductive Causation)
• IC algorithm (Pearl)
– Based on variable dependencies;
– Find all pairs of variables that are dependent
of each other (applying standard statistical
method on the database);
– Eliminate (as much as possible) indirect
dependencies;
– Determine directions of dependencies;
Comparing abduction, deduction and
induction
• Deduction: major premise:
minor premise:
conclusion:
• Abduction: rule:
observation:
explanation:
• Induction: case:
observation:
hypothesized rule:
All balls in the box are black
These balls are from the box
These balls are black
All balls in the box are black
These balls are black
These balls are from the box
These balls are from the box
These balls are black
All ball in the box are black
Induction: from specific cases to general rules;
Abduction and deduction:
both from part of a specific case to other part of
the case using general rules (in different ways)
Source from httpwww.csee.umbc.edu/~ypeng/F02671/lecture-notes/Ch15.ppt
A => B
A
--------B
A => B
B
------------Possibly A
Whenever
A then B
but not
vice versa
------------Possibly
A => B
IC Algorithm (Cont’d)
• Input:
– P – a stable distribution on a set V of
variables;
• Output:
– A pattern H(P) compatible with P;
Patten: is a partially directed DAG
• some edges are directed and
• some edges are undirected;
IC Algorithm: Step 1
• For each pair of variables a and b in V, search for a set Sab
such that (a╨b | Sab) holds in P – in other words, a and b should be
independent in P, conditioned on Sab .
• Construct an undirected graph G such that vertices a and
b are connected with an edge if and only if no set Sab can
be found.
 Sab
a
╨
b
a
b
a
b
Sab
Not  Sab
IC Algorithm: Step 2
• For each pair of nonadjacent variables a and b with a
common neighbor c, check if c Sab.
• If it is, then continue;
• Else add arrowheads at c
• i.e a→ c ← b
Yes
a
c
b
a
╨
b
C
No
a
c
b
Example
Other causes
of mud
Rain
Mud
Other causes
of mud
Rain
Mud
IC Algorithm Step 3
• In the partially directed graph that results,
orient as many of the undirected edges as
possible subject to two conditions:
– The orientation should not create a new vstructure;
– The orientation should not create a directed cycle;
Rules required to obtaining a
maximally oriented pattern
• R1: Orient b — c into b→c whenever there is
an arrow a→b such that a and c are non
adjacent;
b
c
b
a
b
c
c
Rules required to obtaining a
maximally oriented pattern
• R2: Orient a — b into a→b whenever
there is a chain a→c→b;
a
b
a
a
c
b
b
Rules required to obtaining a
maximally oriented pattern
R3: Orient a — b into a→b whenever there
are two chains a—c→b and a—d→b such
that c and d are nonadjacent;
a
b
a
c
a
b
d
b
Rules required to obtaining a
maximally oriented pattern
R4: Orient a — b into a→b whenever there
are two chains a—c→d and c→d→b such
that c and b are nonadjacent;
a
b
a
a
c
d
c
d
b
b
IC* Algorithm
• Input:
– P, a sampled distribution;
• Output:
– core(P), a marked pattern;
Marked Pattern:Four types of
edges
IC* Algorithm: Step 1
For each pair of variables a and b, search
for a set Sab such that a and b are
independent in P, conditioned on Sab. If
there is no such Sab, place an undirected
link between the two variables, a – b.
IC* Algorithm: Step 2
• For each pair of nonadjacent variables a and b
with a common neighbor c, check if cSab
– If it is, then continue;
– If it is not, then add arrow heads pointing at c (i.e. a 
c  b).
• In the partially directed graph that results, add
(recursively) as many arrowheads as possible,
and mark as many edges as possible, according
to the following two rules:
IC* Algorithm: Rule 1
• R1: For each pair of non-adjacent nodes a and b with
a common neighbor c, if the link between a and c has
an arrow head into c and if the link between c and b
has no arrowhead into c, then add an arrow head on
the link between c and b pointing at b and mark that
link to obtain c –* b;
a
a
c
b
*
b
c
IC* Algorithm: Rule 2
• R2: If a and b are adjacent and there is a directed
path (composed strictly of marked links) from a to
b, then add an arrowhead pointing toward b on
the link between a and b;