# ABC methods for model choice in Gibbs random fields

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1 ABC methods for model choice in Gibbs random fields Jean-Michel Marin Institut de Mathématiques et Modélisation Université Montpellier 2 Joint work with Aude Grelaud, Christian Robert, François Rodolphe, Jean-François Taly ABC in Paris, 26/06/2009 Page 1

2 We consider a finite set of sites S = {1,, n}. At each site i S, we observe x i X i where X i is a finite set of states. are said neigh- We also consider an undirected graph G: the sites i and i bours, if there is a vertex between i and i. A clique c is a subset of S where all elements are mutual neighbours (Daroch, 1980). We denote by C the set of all cliques of the undirected graph G. ABC in Paris, 26/06/2009 Page 2

3 Gibbs Random Fields (GRFs) are probabilistic models associated with densities { f(x) = 1 Z exp{ U(x)} = 1 Z exp } U c (x), c C where U(x) = c C U c(x) is the potential and Z is the corresponding normalising constant { } Z = x X exp c C U c (x). If the density f of a Markov Random Field (MRF) is everywhere positive, then the Hammersley-Clifford theorem establishes that there exists a GRF representation of this MRF (Besag, 1974). ABC in Paris, 26/06/2009 Page 3

4 We consider here GRF with potential U(x) = θ T S(x) where θ R p is a scale parameter, S( ) is a function taking values in R p. S(x) is defined on the cliques of the neighbourhood system in that S(x) = c C S c(x). In that case, we have f(x θ) = 1 Z θ exp{θ T S(x)}, the normalising constant Z θ now depends on the scale parameter θ. ABC in Paris, 26/06/2009 Page 4

5 GRF are used to model the dependency within spatially correlated data, with applications in epidemiology and image analysis, among others (Rue and Held, 2005). They often use a Potts model defined by a sufficient statistic S taking values in R in that S(x) = i i I {xi =x i }, where i i pairs. indicates that the summation is taken over all the neighbour X i = {1,, K}, K = 2 corresponding to the Ising model, and θ is a scalar. S( ) monitors the number of identical neighbours over X. ABC in Paris, 26/06/2009 Page 5

6 In most realistic settings, the summation Z θ = x X exp{θ T S(x)} involves too many terms to be manageable. Selecting a model with sufficient statistic S 0 versus a model with sufficient statistics S 1 relies on the Bayes factor / BF m0 /m 1 (x) = exp{θ T 0 S 0 (x)}/z θ0,0π 0 (dθ 0 ) exp{θ T 1 S 1 (x)}/z θ1,1π 1 (dθ 1 ) This quantity is not easily computable. ABC in Paris, 26/06/2009 Page 6

7 For a fixed neighbourhood or model, the unavailability of Z θ complicates inference on the scale parameter θ. The difficulty is increased manifold when several neighbourhood structures are under comparison. We propose a procedure based on an ABC algorithm aimed at selecting a model. We consider the toy example of an iid sequence [with trivial neighbourhood structure] tested against a Markov chain model [with nearest neighbour structure]. ABC in Paris, 26/06/2009 Page 7

8 In a model choice perspective, we face M Gibbs random fields in competition. Each model m is associated with sufficient statistic S m (0 m M 1), i.e. with corresponding likelihood f m (x θ m ) = exp { θ T ms m (x) } / Z θm,m, where θ m Θ m and Z θm,m is the unknown normalising constant. The choice between those models is driven by the posterior probabilities of the models. ABC in Paris, 26/06/2009 Page 8

9 We consider an extended parameter space Θ = M 1 m=0 {m} Θ m that includes the model index M, We define a prior distribution on the model index π(m = m) as well as a prior distribution on the parameter conditional on the value m of the model index, π m (θ m ), defined on the parameter space Θ m. The computational target is thus the model posterior probability P(M = m x) f m (x θ m )π m (θ m ) dθ m π(m = m), Θ m the marginal of the posterior distribution on (M, θ 0,..., θ M 1 ) given x. ABC in Paris, 26/06/2009 Page 9

10 If S(x) is a sufficient statistic for the joint parameters (M, θ 0,..., θ M 1 ), P(M = m x) = P(M = m S(x)). Each model has its own sufficient statistic S m ( ). Then, for each model, the vector of statistics S( ) = (S 0 ( ),..., S M 1 ( )) is obviously sufficient. ABC in Paris, 26/06/2009 Page 10

11 We have shown that the statistic S(x) is also sufficient for the joint parameters (M, θ 0,..., θ M 1 ). That the concatenation of the sufficient statistics of each model is also a sufficient statistic for the joint parameters is a property that is specific to Gibbs random field models. When we consider M models from generic exponential families, this property of the concatenated sufficient statistic rarely holds. ABC in Paris, 26/06/2009 Page 11

12 ABC algorithm for model choice (ABC-MC) 1. Generate m from the prior π(m = m). 2. Generate θm from the prior π m ( ). 3. Generate x from the model f m ( θm ). 4. Compute the distance ρ(s(x 0 ), S(x )). 5. Accept (θm, m ) if ρ(s(x 0 ), S(x )) ɛ, otherwise, start again in 1. ABC in Paris, 26/06/2009 Page 12

13 Simulating a data set x from f m ( θm ) at step 3 is non-trivial for GRFs (Møller and Waagepetersen, 2003). It is often possible to use a Gibbs sampler updating one clique at a time conditional on the others. This algorithm results in an approximate generation from the joint posterior distribution π { (M, θ 0,..., θ M 1 ) ρ(s(x 0 ), S(x )) ɛ }. When it is possible to achieve ɛ = 0, the algorithm is exact since S is a sufficient statistic. ABC in Paris, 26/06/2009 Page 13

14 Once a sample of N values of (θ i m i, m i ) (1 i N) is generated from this algorithm, a standard Monte Carlo approximation of the posterior probabilities is provided by the empirical frequencies of visits to the model, namely P(M = m x 0 ) = {m i = m} / N, where {m i = m} denotes the number of simulated m i s equal to m. ABC in Paris, 26/06/2009 Page 14

15 BF m0 /m 1 (x 0 ) = P(M = m 0 x 0 ) P(M = m 1 x 0 ) π(m = m 1 ) π(m = m 0 ) BF m0 /m 1 (x 0 ) = {mi = m 0 } {m i = m 1 } π(m = m 1) π(m = m 0 ), This estimate is only defined when {m i = m 1 } 0. To bypass this difficulty, the substitute BF m0 /m 1 (x 0 ) = 1 + {mi = m 0 } 1 + {m i = m 1 } π(m = m 1) π(m = m 0 ) is particularly interesting because we can evaluate its bias. ABC in Paris, 26/06/2009 Page 15

16 We set N 0 = {m i = m 0 } and N 1 = {m i = m 1 }. If π(m = m 1 ) = π(m = m 0 ), then N 1 is a binomial B(N, ρ) random variable with probability ρ = (1 + BF m0 /m 1 (x 0 )) 1 and E [ ] N0 + 1 N = BF m0 /m 1 (x 0 ) + 1 ρ(n + 1) N + 2 ρ(n + 1) (1 ρ)n+1. The bias of BF m0 /m 1 (x 0 ) is {1 (N + 2)(1 ρ) N+1 }/(N + 1)ρ, which goes to zero as N goes to infinity. BF m0 /m 1 (x 0 ) can be seen as the ratio of the posterior means on the model probabilities under a Dir(1,, 1) prior. ABC in Paris, 26/06/2009 Page 16

17 BF m0 /m 1 (x 0 ) suffers from a large variance when BF m0 /m 1 (x 0 ) is very large since. When P(M = m 1 x 0 ) is very small, {m i = m 1 } is most often equal to zero. We can used a reweighting scheme. If the choice of m in the ABC algorithm is driven by the probability distribution P(M = m 1 ) = ϱ = 1 P(M = m 0 ) rather than by π(m = m 1 ) = 1 π(m = m 0 ), the value of {m i = m 1 } can be increased and later corrected by considering instead BF m0 /m 1 (x 0 ) = 1 + {mi = m 0 } 1 + {m i = m 1 } ϱ 1 ϱ. ABC in Paris, 26/06/2009 Page 17

18 Two step ABC: If a first run of the ABC algorithm exhibits a very large value of BF m0 /m 1 (x 0 ), the estimate BF m0 /m 1 (x 0 ) produced by a second run with / ϱ 1 ˆP(M = m 1 x 0 ) will be more stable than the original BF m0 /m 1 (x 0 ). ABC in Paris, 26/06/2009 Page 18

19 Results on a toy example: Our first example compares an iid Bernoulli model with a two-state firstorder Markov chain. Both models are special cases of GRF, the first one with a trivial neighbourhood structure and the other one with a nearest neighbourhood structure. Furthermore, the normalising constant Z θm,m can be computed in closed form, as well as the posterior probabilities of both models. ABC in Paris, 26/06/2009 Page 19

20 We consider a sequence x = (x 1,.., x n ) of binary variables. Under model M = 0, the GRF representation of the Bernoulli distribution B(exp(θ 0 )/{1 + exp(θ 0 )}) is ( f 0 (x θ 0 ) = exp n θ 0 i=1 I {xi =1}) / {1 + exp(θ 0 )} n. For θ 0 U( 5, 5), the posterior probability of this model is available since the marginal when S 0 (x) = s 0 (s 0 0) is given by 1 10 s 0 1 k=0 ( ) s0 1 ( 1) s 0 1 k k n 1 k [ (1 + e 5 ) k n+1 (1 + e 5 ) k n+1]. ABC in Paris, 26/06/2009 Page 20

21 Model M = 1 is chosen as a Markov chain. We assume a uniform distribution on x 1 and ( f 1 (x θ 1 ) = 1 n 2 exp / I {xi =x i 1 }) {1 + exp(θ 1 )} n 1. θ 1 i=2 For θ 1 U(0, 6), the posterior probability of this model is once again available, the likelihood being of the same form as when M = 0. ABC in Paris, 26/06/2009 Page 21

22 We simulated 2, 000 datasets x 0 = (x 1,, x n ) with n = 100 under each model, using parameters simulated from the priors. For each of those 2, 000 datasets x 0, the ABC-MC algorithm was run for loops, meaning that sets (m, θ m, x ) were exactly simulated from the joint distribution. A random number of those were accepted when S(x ) = S(x 0 ). (In the worst case scenario, the number of acceptances was 12!) ABC in Paris, 26/06/2009 Page 22

23 ^ P(M=0 x) ^ P(M=0 x) P(M=0 x) P(M=0 x) Figure 1: (left) Comparison of the true P(M = 0 x 0 ) with P(M = 0 x 0 ) over 2, 000 simulated sequences and proposals from the prior. The red line is the diagonal. (right) Same comparison when using a tolerance ɛ corresponding to the 1% quantile on the distances. ABC in Paris, 26/06/2009 Page 23

24 BF ^ BF ^ BF 01 BF 01 Figure 2: (left) Comparison of the true BF m0 /m 1 (x 0 ) with BF m0 /m 1 (x 0 ) (in logarithmic scales) over 2, 000 simulated sequences and proposals from the prior. The red line is the diagonal. (right) Same comparison when using a tolerance corresponding to the 1% quantile on the distances. ABC in Paris, 26/06/2009 Page 24

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