jmlr jmlr2013 jmlr2013-121 knowledge-graph by maker-knowledge-mining

121 jmlr-2013-Variational Inference in Nonconjugate Models


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Author: Chong Wang, David M. Blei

Abstract: Mean-field variational methods are widely used for approximate posterior inference in many probabilistic models. In a typical application, mean-field methods approximately compute the posterior with a coordinate-ascent optimization algorithm. When the model is conditionally conjugate, the coordinate updates are easily derived and in closed form. However, many models of interest—like the correlated topic model and Bayesian logistic regression—are nonconjugate. In these models, mean-field methods cannot be directly applied and practitioners have had to develop variational algorithms on a case-by-case basis. In this paper, we develop two generic methods for nonconjugate models, Laplace variational inference and delta method variational inference. Our methods have several advantages: they allow for easily derived variational algorithms with a wide class of nonconjugate models; they extend and unify some of the existing algorithms that have been derived for specific models; and they work well on real-world data sets. We studied our methods on the correlated topic model, Bayesian logistic regression, and hierarchical Bayesian logistic regression. Keywords: variational inference, nonconjugate models, Laplace approximations, the multivariate delta method

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Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 EDU Department of Computer Science Princeton University Princeton, NJ, 08540, USA Editor: Neil Lawrence Abstract Mean-field variational methods are widely used for approximate posterior inference in many probabilistic models. [sent-6, score-0.85]

2 In this paper, we develop two generic methods for nonconjugate models, Laplace variational inference and delta method variational inference. [sent-11, score-1.899]

3 Our methods have several advantages: they allow for easily derived variational algorithms with a wide class of nonconjugate models; they extend and unify some of the existing algorithms that have been derived for specific models; and they work well on real-world data sets. [sent-12, score-0.976]

4 Keywords: variational inference, nonconjugate models, Laplace approximations, the multivariate delta method 1 Introduction Mean-field variational inference lets us efficiently approximate posterior distributions in complex probabilistic models (Jordan et al. [sent-14, score-2.051]

5 ) For such models, which are called conditionally conjugate models, it is easy to derive a coordinate ascent algorithm that optimizes the parameters of the variational distribution (Beal, 2003; Bishop, 2006). [sent-29, score-0.803]

6 , 2010), which allow practitioners to define models of their data and immediately approximate the corresponding posterior with variational inference. [sent-33, score-0.671]

7 Such nonconjugate models1 include Bayesian logistic regression (Jaakkola and Jordan, 1997), Bayesian generalized linear models (Wells, 2001), discrete choice models (Braun and McAuliffe, 2010), Bayesian item response models (Clinton et al. [sent-35, score-0.769]

8 , 2004; Fox, 2010), and nonconjugate topic models (Blei and Lafferty, 2006, 2007). [sent-36, score-0.677]

9 Using variational inference in these settings requires algorithms tailored to the specific model at hand. [sent-37, score-0.769]

10 In this paper we develop two approaches to mean-field variational inference for a large class of nonconjugate models. [sent-40, score-1.205]

11 Formed another way, it is equivalent to using a multivariate delta approximation (Bickel and Doksum, 2007) of the variational objective. [sent-48, score-0.715]

12 Our methods significantly expand the class of models for which mean-field variational inference can be easily applied. [sent-52, score-0.798]

13 We studied our algorithms with three nonconjugate models: Bayesian logistic regression (Jaakkola and Jordan, 1997), hierarchical logistic regression (Gelman and Hill, 2007), and the correlated topic model (Blei and Lafferty, 2007). [sent-53, score-1.073]

14 Further, we found that Laplace variational inference usually outperforms delta method variational inference, both in terms of computation time and the fidelity of the approximate posterior. [sent-55, score-1.475]

15 There have been other efforts to examine generic variational inference in nonconjugate models. [sent-58, score-1.205]

16 (2012a) proposed a variational inference approach using stochastic search for nonconjugate models, approximating the intractable integrals with Monte Carlo methods. [sent-60, score-1.205]

17 (2012) proposed a nonparametric variational inference algorithm, which can be applied to nonconjugate models. [sent-62, score-1.205]

18 1006 VARIATIONAL I NFERENCE IN N ONCONJUGATE M ODELS Laplace approximations have been used in approximate inference in more complex models, though not in the context of mean-field variational inference. [sent-71, score-0.8]

19 Here we want to use them for variational inference, in a method that can be applied to a wider range of nonconjugate models. [sent-76, score-0.976]

20 Finally, we note that the delta method was first used in variational inference by Braun and McAuliffe (2010) in the context of the discrete choice model. [sent-77, score-0.923]

21 In Section 2 we review mean-field variational inference and define the class of nonconjugate models to which our algorithms apply. [sent-80, score-1.255]

22 In Section 3, we derive Laplace and delta-method variational inference and present our full algorithm for nonconjugate inference. [sent-81, score-1.205]

23 In variational inference, we approximate the posterior by positing a simple family of distributions over the latent variables q(θ, z) and then finding the member of that family which minimizes the Kullback-Leibler (KL) divergence to the true posterior (Jordan et al. [sent-88, score-0.804]

24 3 In this section we review variational inference and discuss mean-field variational inference for the class of conditionally conjugate models. [sent-90, score-1.65]

25 We then define a wider class of nonconjugate models for which mean-field variational inference is not as easily applied. [sent-91, score-1.255]

26 In the next section, we derive algorithms for performing mean-field variational inference in this larger class of models. [sent-92, score-0.748]

27 1 Mean-field Variational Inference Mean-field variational inference is simplest and most widely used variational inference method. [sent-94, score-1.496]

28 In mean-field variational inference we posit a fully factorized variational family, q(θ, z) = q(θ)q(z). [sent-95, score-1.267]

29 In this paper, we focus on mean-field variational inference where we minimize the KL divergence to the posterior. [sent-97, score-0.748]

30 We note that there are other kinds of variational inference, with more structured variational distributions or with alternative objective functions (Wainwright and Jordan, 2008; Barber, 2012). [sent-98, score-1.066]

31 In this paper, we use “variational inference” to indicate mean-field variational inference that minimizes the KL divergence. [sent-99, score-0.748]

32 Under the standard variational theory, minimizing the KL divergence between q(θ, z) and the posterior p(θ, z|x) is equivalent to maximizing a lower bound of the log marginal likelihood of the observed data x. [sent-103, score-0.689]

33 These conditions lead to the traditional coordinate ascent algorithm for variational inference. [sent-109, score-0.612]

34 Many applications of variational inference have been developed for this type of model (Bishop, 1999; Attias, 2000; Beal, 2003). [sent-122, score-0.769]

35 That setting arises in many practical models and does not permit closed-form updates or easy calculation of the variational objective. [sent-124, score-0.603]

36 We will develop generic variational inference algorithms for a wide class of nonconjugate models. [sent-125, score-1.205]

37 Traditional variational or Gibbs sampling methods cannot be easily used because the normal prior on the parameters θ is not conjugate to the Dirichlet(exp{θ}) likelihood. [sent-165, score-0.678]

38 We will develop two variational inference algorithms for this class: Laplace variational inference and delta method variational inference. [sent-176, score-2.19]

39 Both use coordinate ascent to optimize the variational parameters, iterating between updating q(θ) and q(z). [sent-177, score-0.657]

40 They differ in how they update the variational distribution of the nonconjugate variable q(θ). [sent-178, score-1.08]

41 In delta method variational inference, we apply Taylor approximations to approximate the variational objective in Equation 3 and then derive the corresponding updates. [sent-181, score-1.293]

42 Formed another way, it is equivalent to using a multivariate delta approximation (Bickel and Doksum, 2007) of the variational objective function. [sent-184, score-0.743]

43 The variational distribution of the nonconjugate variable q(θ) is a Gaussian; the variational distribution of the conjugate variable q(z) is in the same family as p(z | η(θ)). [sent-186, score-1.736]

44 Our algorithms are coordinate ascent algorithms, where we iterate between updating the nonconjugate variational distribution q(θ) and updating the conjugate variational distribution q(z). [sent-194, score-1.794]

45 Now we describe how we use Laplace approximations as part of a variational inference algorithm for more complex models. [sent-220, score-0.767]

46 2 Laplace Updates in Variational Inference We adapt the idea behind Laplace approximations to update the variational distribution q(θ). [sent-223, score-0.623]

47 The update in Equation 13 can be used in a coordinate ascent algorithm for a nonconjugate model. [sent-239, score-0.616]

48 2 Delta Method Variational Inference In Laplace variational inference, the variational distribution q(θ) Equation 13 is solely a function of ˆ θ, the maximum of f (θ) in Equation 11. [sent-253, score-1.057]

49 We approximate the variational objective L in Equation 3 and then optimize that approximation. [sent-256, score-0.6]

50 We set the variational distribution q(θ) to be a Gaussian N (µ, Σ), where the parameters are free variational parameters fit to optimize the variational objective. [sent-258, score-1.596]

51 In the coordinate update of q(θ), this is the function we optimize with respect to its variational parameters {µ, Σ}. [sent-266, score-0.641]

52 Delta method variational inference optimizes this objective in the coordinate update of q(θ) . [sent-286, score-0.896]

53 Note this is more expensive than Laplace variational inference because optimizing Equation 16 requires the third derivative ▽3 f (θ). [sent-288, score-0.748]

54 Braun and McAuliffe (2010) were the first to use the delta method in a variational inference algorithm, developing this technique for the discrete choice model. [sent-289, score-0.923]

55 While Laplace inference required the digamma function and log Γ function, delta method inference will further require the trigamma function. [sent-294, score-0.704]

56 We now turn to the update for the variational distribution of the conjugate variable q(z). [sent-297, score-0.741]

57 Recall that η(θ) maps the nonconjugate variable θ to the natural parameter of the conjugate variable z. [sent-307, score-0.613]

58 ) Using delta method variational inference to update q(θ), the update for q(z) is identical to that in Laplace variational inference. [sent-314, score-1.574]

59 5: Figure 1: Nonconjugate variational inference Setting the partial gradient ∂L (q(z))/∂q(z) = 0 gives the same optimal q(z) of Equation 5. [sent-326, score-0.748]

60 Computing this update reduces to the approach for Laplace variational inference in Equation 17. [sent-327, score-0.814]

61 To implement nonconjugate inference we need this update for q(z) and the definition of f (·) in Equation 14. [sent-333, score-0.752]

62 4 Nonconjugate Variational Inference We now present the full algorithm for nonconjugate variational inference. [sent-335, score-0.976]

63 Recall that the variational distribution of the nonconjugate variable is a Gaussian q(θ | µ, Σ); the variational distribution of the conjugate variable is q(z | φ), where φ is a natural parameter in the same family as p(z | η(θ)). [sent-337, score-1.736]

64 In either Laplace or delta method inference, we have reduced deriving variational updates for complicated nonconjugate models to mechanical work— calculating derivatives and calling a numerical optimization library. [sent-345, score-1.235]

65 We note that Laplace inference is simpler to derive because it only requires second derivatives of the function in Equation 11; 1015 WANG AND B LEI Figure 2: The approximate variational objective from Equation 19 goes up as a function of the iteration. [sent-346, score-0.809]

66 4 Example Models We have described a generic algorithm for approximate posterior inference in nonconjugate models. [sent-362, score-0.788]

67 In this section we derive this algorithm for several nonconjugate models from the research literature: the correlated topic model (Blei and Lafferty, 2007), Bayesian logistic regression (Jaakkola and Jordan, 1997), and hierarchical Bayesian logistic regression (Gelman and Hill, 2007). [sent-363, score-1.123]

68 The nonconjugate variable is θ; the conjugate variable is the collection z = z1:N ; the observation is the collection of words x = x1:N . [sent-369, score-0.663]

69 model, we identify the variables—the nonconjugate variable θ, conjugate variable z, and observations x—and we calculate f (θ) from Equation 11. [sent-370, score-0.613]

70 )The nonconjugate variable is the vector of coefficients θm , the conjugate variable is the collection of observed classes for each data point, zm = zm,1:N . [sent-405, score-0.638]

71 This calculation is important in two contexts: it is used when forming predictions about new data; and it is used as a subroutine in the variational expectation maximization algorithm for fitting the topics and logistic normal parameters (mean µ0 and covariance Σ0 ) with maximum likelihood. [sent-412, score-0.719]

72 In terms of the earlier notation, the nonconjugate variable is the topic proportions θ, the conjugate variable is the collection of topic assignments z = z1:N , and the observation is the collection of words x = x1:N . [sent-417, score-1.047]

73 The variational distribution for the topic proportions θ is Gaussian, q(θ) = N (µ, Σ); the variational distribution for the topic assignments is discrete, q(z) = ∏n q(zn | φn ) where each φn is a distribution over K elements. [sent-418, score-1.479]

74 In delta method inference, as in Braun and McAuliffe (2010), we restrict the variational covariance Σ to be diagonal to simplify the derivative of Equation 16. [sent-419, score-0.694]

75 Besides the CTM, this approach can be adapted to a variety of nonconjugate topic models, including the topic evolution model (Xing, 2005), Dirichlet-multinomial regression (Mimno and McCallum, 2008), dynamic topic models (Blei and Lafferty, 2006; Wang et al. [sent-422, score-1.071]

76 Using Laplace variational inference, our approach recovers the standard Laplace approximation for Bayesian logistic regression (Bishop, 2006). [sent-442, score-0.702]

77 m m As for the CTM, we use nonconjugate inference as a subroutine in a variational EM algorithm (where the M step is regularized). [sent-453, score-1.205]

78 1019 WANG AND B LEI 5 Empirical Study We studied nonconjugate variational inference with correlated topic models and Bayesian logistic regression. [sent-459, score-1.604]

79 We found that nonconjugate inference is more accurate than the existing methods tailored to specific models. [sent-460, score-0.686]

80 Between the two nonconjugate inference algorithms, we found that Laplace inference is faster and more accurate than delta method inference. [sent-461, score-1.09]

81 1 The Correlated Topic Model We studied Laplace inference and delta method inference in the CTM. [sent-463, score-0.633]

82 In the E-step we perform approximate posterior inference with each document, estimating its topic proportions and topic assignments. [sent-474, score-0.715]

83 We fit models with different kinds of Esteps, using both of the nonconjugate inference methods from Section 3 and the original approach of Blei and Lafferty (2007). [sent-476, score-0.736]

84 To initialize nonconjugate inference we set the variational mean parameter µ = 0 for log topic proportions θ and computed the corresponding updates for the topic assignments z. [sent-477, score-1.694]

85 With nonconjugate inference in the E-step, variational EM approximately optimizes a bound on the marginal probability of the observed data. [sent-479, score-1.223]

86 We split each held-out document in to two halves (w1 , w2 ) and form the approximate posterior log topic proportions qw1 (θ) using one of the approximate inference algorithms and the first half of the document w1 . [sent-486, score-0.775]

87 Figure 5 (a) indicates that the approximate bounds from nonconjugate inference generally go up as the number of topics increases. [sent-493, score-0.769]

88 Finally, note that Laplace variational inference was always better than both other algorithms. [sent-501, score-0.748]

89 Given a test-case input t with label z, we compute the log predictive likelihood, log p(z | µ,t) = z1 log σ(µ⊤t) + z2 log σ(−µ⊤t), where µ is the mean of variational distribution q(θ) = N (µ, Σ). [sent-521, score-0.866]

90 1021 WANG AND B LEI Figure 5: Laplace variational inference is “Lap-Var”; delta method variational inference is “DeltaVar”; Blei and Lafferty’s method is “BL. [sent-538, score-1.671]

91 Laplace inference and delta method inference gave slightly better accuracy than Jaakkola and Jordan’s method, and much 1022 VARIATIONAL I NFERENCE IN N ONCONJUGATE M ODELS Figure 6: In this figure, we set the number of topics as K = 60. [sent-550, score-0.683]

92 With predictive likelihood, Laplace variational inference in the hierarchical model is significantly better than all other approaches. [sent-608, score-0.864]

93 6 Discussion We developed Laplace and delta method variational inference, two strategies for variational inference in a large class of nonconjugate models. [sent-609, score-1.899]

94 ) We compared Laplace inference, delta inference and Jaakkola and Jordan’s (1996) method in three settings: separate logistic regression models for each school, a pooled logistic regression model for all schools, and the hierarchical logistic regression model in Section 4. [sent-636, score-1.033]

95 Similar to the main paper, we use mean-field variational inference (Jordan et al. [sent-654, score-0.748]

96 1) to approximate it, although delta method variational inference (Section 3. [sent-663, score-0.956]

97 With this notation, f (θ) = η(θ)⊤ Eq(z) [t(z)] − 1 (θ − µ0 )⊤ Σ−1 (θ − µ0 ), 0 2 where Eq(z) [t(z)] is the expected word counts of each topic under the variational distribution q(z). [sent-682, score-0.739]

98 In delta method variational inference, we also need to compute the gradient of Trace ▽2 f (θ)Σ = − ∑K πk Σkk + πT Σπ ∑K Eq(z) [t(z)] k − Trace(Σ−1 Σ). [sent-687, score-0.694]

99 For delta variational inference, we also need the gradient for Trace ▽2 f (θ)Σ . [sent-708, score-0.694]

100 A generalized mean field algorithm for variational inference in exponential families. [sent-990, score-0.779]


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