New Complexity Results for MAP in Bayesian
Networks
Cassio P.de Campos
Dalle Molle Institute for Articial Intelligence
Switzerland
IJCAI,2011
Bayesian nets
I
Directed acyclic graph (DAG) with nodes associated to
(categorical) random variables;
I
Collection of conditional probabilities p(X
i
j
i
) where
i
denotes the parents of X
i
in the graph (
i
may be empty);
I
Every variable is conditionally independent of its
nondescendants given its parents (Markov condition).
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#1
Bayesian nets
I
In other words,it is a compact way based on (in)dependence
relations to represent a joint probability distribution.
p(X
1
;:::;X
n
) =
Y
i
p(X
i
j
i
)
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#2
Belief Updating
I
BU:given a set of queried variables X and their states x,
evidence variables E and their states e,compute
p(X = xjE = e).
p(ajd;e) =
p(a;d;e)
p(d;e)
=
P
b;c
p(a;b;c;d;e)
P
a;b;c
p(a;b;c;d;e)
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#3
Belief Updating  Decision version
I
In terms of complexity,we can restrict ourselves to the
computation of p(X = x;E = e).
I
DBU:given a rational ,a set of queried variables X and
their states x,evidence variables E and their states e,decide
whether p(x;e) > .
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#4
(Partial) Maximum a Posterior (MAP)
I
MAP:given a set of queried variables X,evidence variables E
and their states e,compute
argmax
x
p(xje) = argmax
x
p(x;e).
argmax
a
p(a;d;e) = argmax
a
X
b;c
p(a;b;c;d;e):
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#5
(Partial) Maximum a Posterior (MAP)  Decision version
I
DMAP:given a rational ,a set of queried variables X,
evidence variables E and their states e,decide whether
max
x
p(x;e) > .
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#6
Restricting Treewidth and Maximum cardinality
I
MAPzw and DMAPzw:same problems as before,but
with two restrictions:z is a bound on the cardinality of any
variable in the network,and w is a bound on the treewidth of
the network.(The same denition can be used for the BU
problem,which becomes BUzw and DBUzw.)
I
In order to express no bound,we use the symbol 1.E.g.
DMAP11is the problem as dened earlier,and
DMAP1w has a bound for treewidth,but not for for
cardinality.
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#7
Previous results
Complexity of this problems had been studied before,including the
case of bounded treewidth.
I
DBU11is PPcomplete,while DBU1w is in P.In fact,
limiting the cardinality does not help:DBU21 is still
PPcomplete [Littman et al.2001].The functional versions
are similar and discussed in [Roth 1996].
I
DMAP11is NP
PP
complete,while DMAP1w is
NPcomplete [Park & Darwiche 2004].
I
MAP1w is also shown not to be in PolyAPX [Park &
Darwiche 2004].(Unless P=NP) It is shown that there is no
polynomial time approximation that can achieve a 2
b
"
factor
approximation,for 0 <"< 1,b is the length of the input.
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#8
...but cardinality has been neglected so far
This paper presents new results for MAP that take cardinality into
consideration.
I
DMAP22 remains NPcomplete (trick reduction from
PARTITION).
I
This includes binary polytrees.
I
DMAP11 remains NPcomplete (reduction from
MAX2SAT using a naivelike structure) and DMAP51 is
NPcomplete too (reduction from PARTITION using an
HMMlike structure).
I
This includes even simple trees.
I
It is NPhard to approximate MAP11 to any factor 2
b
"
(the
construction comes from the naivelike structure,and uses
similar arguments as in [Park & Darwiche 2004]).
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#9
Decision problems
DMAP11
DMAP21
DBU11
DBU21
DMAP11
DMAP12
DMPE21
DMPE11
DMAP22
DMAP51
DBU1w
DMPE1w
NP
PP
PP
NP
P
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#10
...and there is (some) hope...
MAPzw is hard,but has a FPTAS!
I
We develop a Fully Polynomialtime Approximation Scheme
for MAP when both treewidth and cardinality are bounded.
I
The idea is to compute all possible candidates and propagate
them as in a BU inference,but keeping the number of
candidates bounded by a polynomial in the length of the input
(following ideas from [Papadimitriou and Yannakakis,2000]).
I
Previous inapproximability results are not contradicted:they
had used variables with high cardinality.
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#11
Functional problems
MAP11
MAP21
MPE11
MPE21
MAP12
MAP11
MAP1w
MAPzw
MPE1w
BU1w
?
NPO
expAPX
polyAPX
FPTAS
FP
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#12
Conclusions
This paper targets on understanding better the computational
complexity of MAP.
I
The problem is shown to remain hard in binary polytrees and
trees with bounded cardinality.
I
The problem is shown to be not approximable even in trees
(without cardinality restrictions).
I
An FPTAS is devised when both treewidth and cardinality are
bounded.
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#13
Thanks
Thank you for your attention.Further questions:cassiopc@acm.org
Work partially supported by
I
Project Computational Life Sciences  Ticino in Rete,
Switzerland.
I
Grant from the Swiss NSF n.200020
134759/1.
Cassio P.de Campos
New Complexity Results for MAP in Bayesian Networks Slide#14
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