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168 nips-2005-Rodeo: Sparse Nonparametric Regression in High Dimensions


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Author: Larry Wasserman, John D. Lafferty

Abstract: We present a method for nonparametric regression that performs bandwidth selection and variable selection simultaneously. The approach is based on the technique of incrementally decreasing the bandwidth in directions where the gradient of the estimator with respect to bandwidth is large. When the unknown function satisfies a sparsity condition, our approach avoids the curse of dimensionality, achieving the optimal minimax rate of convergence, up to logarithmic factors, as if the relevant variables were known in advance. The method—called rodeo (regularization of derivative expectation operator)—conducts a sequence of hypothesis tests, and is easy to implement. A modified version that replaces hard with soft thresholding effectively solves a sequence of lasso problems. 1

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Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 The approach is based on the technique of incrementally decreasing the bandwidth in directions where the gradient of the estimator with respect to bandwidth is large. [sent-2, score-0.731]

2 When the unknown function satisfies a sparsity condition, our approach avoids the curse of dimensionality, achieving the optimal minimax rate of convergence, up to logarithmic factors, as if the relevant variables were known in advance. [sent-3, score-0.265]

3 The method—called rodeo (regularization of derivative expectation operator)—conducts a sequence of hypothesis tests, and is easy to implement. [sent-4, score-0.484]

4 A modified version that replaces hard with soft thresholding effectively solves a sequence of lasso problems. [sent-5, score-0.323]

5 1 Introduction Estimating a high dimensional regression function is notoriously difficult due to the “curse of dimensionality. [sent-6, score-0.049]

6 Then if m is in W2 (c), the d-dimensional Sobolev ball of order two and radius c, it is well known that lim inf n4/(4+d) inf sup R(mn , m) > 0 , (1) n→∞ mn m∈W2 (c) b where R(mn , m) = Em (mn (x) − m(x))2 dx is the risk of the estimate mn constructed on a sample of size n (Gy¨ rfi et al. [sent-15, score-0.203]

7 Under this sparseness assumption we can hope to achieve the better minimax convergence rate of n−4/(4+r) if the r relevant variables can be isolated. [sent-24, score-0.164]

8 Thus, we are faced with the problem of variable selection in nonparametric regression. [sent-25, score-0.187]

9 A large body of previous work has addressed this fundamental problem, which has led to a variety of methods to combat the curse of dimensionality. [sent-26, score-0.057]

10 (2005) use likelihood basis pursuit, essentially the lasso adapted to the spline setting. [sent-31, score-0.106]

11 1984) and MARS (Friedman 1991) effectively perform variable selection as part of their function fitting. [sent-33, score-0.123]

12 (2005) use independence testing for variable selection and B¨ hlmann and Yu (2005) introduced a boosting approach. [sent-35, score-0.13]

13 Indeed, the theoretical analysis of sparse parametric estimators such as the lasso (Tibshirani 1996) is difficult, and only recently has significant progress been made on this front (Donoho 2004; Fu and Knight 2000). [sent-38, score-0.126]

14 In this paper we present a new approach to sparse nonparametric function estimation that is both computationally simple and amenable to theoretical analysis. [sent-39, score-0.114]

15 It is based on the idea that bandwidth and variable selection can be simultaneously performed by computing the infinitesimal change in a nonparametric estimator as a function of the smoothing parameters, and then thresholding these derivatives to effectively get a sparse estimate. [sent-41, score-0.837]

16 As a simple version of this principle we use hard thresholding, effectively carrying out a sequence of hypothesis tests. [sent-42, score-0.108]

17 A modified version that replaces testing with soft thresholding effectively solves a sequence of lasso problems. [sent-43, score-0.281]

18 The potential appeal of this approach is that it can be based on relatively simple and theoretically well understood nonparametric techniques such as local linear smoothing, leading to methods that are simple to implement and can be used in high dimensional problems. [sent-44, score-0.092]

19 Moreover, we show that the rodeo can achieve near optimal minimax rates of convergence, and therefore circumvents the curse of dimensionality when the true function is indeed sparse. [sent-45, score-0.559]

20 Fix a point x and let mh (x) denote an estimator of m(x) based on a vector of smoothing parameters h = (h1 , . [sent-49, score-0.485]

21 For now, assume that xi is one of the observed data points and that m0 (x) = Yi . [sent-58, score-0.049]

22 If P = (h(t) : 0 ≤ t ≤ 1) is a smooth path through the set of smoothing parameters with h(0) = 0 and h(1) = 1 (or any other fixed, large bandwidth) then 1 1 dM (h(s)) ˙ m(x) = M (0) = M (1) − ds = M (1) − D(s), h(s) ds ds 0 0 where D(h) = ∇M (h) = ∂M ∂M ∂hj , . [sent-60, score-0.276]

23 , ∂hj T ˙ is the gradient of M (h) and h(s) = dh(s) ds is the derivative of h(s) along the path. [sent-63, score-0.104]

24 A biased, low variance estimator of M (1) is m1 (x). [sent-64, score-0.117]

25 An unbiased estimator of D(h) is Z(h) = ∂ mh (x) ∂ mh (x) ,. [sent-65, score-0.765]

26 (2) ˙ Z(s), h(s) ds (3) The naive estimator 1 m(x) = m1 (x) − 0 h2 Start Rodeo path Ideal path Figure 1: The bandwidths for the relevant variables (h2 ) are shrunk, while the bandwidths for the irrelevant variables (h1 ) are kept relatively large. [sent-69, score-0.921]

27 The simplest rodeo algorithm shrinks the bandwidths in discrete steps 1, β, β 2 , . [sent-70, score-0.692]

28 Optimal bandwidth h1 is identically equal to m0 (x) = Yi , which has poor risk since the variance of Z(h) is large for small h. [sent-74, score-0.327]

29 However, our sparsity assumption on m suggests that there should be paths for which D(h) is also sparse. [sent-75, score-0.072]

30 Along such a path, we replace Z(h) with an estimator D(h) that makes use of the sparsity assumption. [sent-76, score-0.156]

31 Our estimate of m(x) is then 1 m(x) = m1 (x) − ˙ D(s), h(s) ds . [sent-77, score-0.065]

32 (4) 0 To implement this idea we need to do two things: (i) we need to find a sparse path and (ii) we need to take advantage of this sparseness when estimating D along that path. [sent-78, score-0.12]

33 The key observation is that if xj is irrelevant, then we expect that changing the bandwidth hj for that variable should cause only a small change in the estimator mh (x). [sent-79, score-1.274]

34 Conversely, if xj is relevant, then we expect that changing the bandwidth hj for that variable should cause a large change in the estimator. [sent-80, score-0.833]

35 Thus, Zj = ∂ mh (x)/∂hj should discriminate between relevant and irrelevant covariates. [sent-81, score-0.431]

36 To simplify the procedure, we can replace the continuum of bandwidths with a discrete set where each hj ∈ B = {h0 , βh0 , β 2 h0 , . [sent-82, score-0.614]

37 Moreover, we can proceed in a greedy fashion by estimating D(h) sequentially with hj ∈ B and setting Dj (h) = 0 when hj < hj , where hj is the first h such that |Zj (h)| < λj (h) for some threshold λj . [sent-86, score-1.523]

38 This greedy version, coupled with the hard threshold estimator, yields m(x) = mb (x). [sent-87, score-0.097]

39 This idea can be implemented using a greedy algorithm, coupled with the hard threshold estimator, to yield a bandwidth selection procedure based on testing. [sent-89, score-0.474]

40 This approach to bandwidth selection is similar to that of Lepski et al. [sent-90, score-0.39]

41 Our approach is also similar to a method of Ruppert (1997) that uses a sequence of decreasing bandwidths and then estimates the optimal bandwidth by estimating the mean squared error as a function of bandwidth. [sent-92, score-0.565]

42 Our greedy approach tests whether an infinitesimal change in the bandwidth from its current setting leads to a significant change in the estimate, and is more easily extended to a practical method in higher dimensions. [sent-93, score-0.415]

43 (2001) focuses on variable selection in multi-index models rather than on bandwidth estimation. [sent-95, score-0.391]

44 3 Rodeo using Local Linear Regression We now present the multivariate case in detail, using local linear smoothing as the basic method since it is known to have many good properties. [sent-96, score-0.067]

45 Let mH (x) denote the local linear estimator of m(x) using bandwidth matrix H. [sent-101, score-0.413]

46 The estimator mH can be written as mH (x) = n i=1 G(Xi , x, h) Yi where 1 T G(u, x, h) = eT (Xx Wx Xx )−1 KH (u − x) (6) 1 (u − x)T is called the effective kernel. [sent-112, score-0.117]

47 We assume that the covariates are random with sampling density f (x), and make the same assumptions as Ruppert and Wand (1994) in their analysis of the bias and variance of local linear regression. [sent-113, score-0.136]

48 In particular, (i) the kernel K has compact support with zero odd moments and uu⊤ K(u) du = ν2 (K)I and (ii) the sampling density f (x) is continuously differentiable and strictly positive. [sent-114, score-0.053]

49 In the version of the algorithm that follows, we take K to be a product kernel and H to be diagonal with elements h = (h1 , . [sent-115, score-0.069]

50 Our method is based on the statistic ∂ mh (x) Zj = = ∂hj where Gj (u, x, h) = Zj = ∂G(u,x,h) . [sent-119, score-0.324]

51 ∂hj n Gj (Xi , x, h)Yi (7) i=1 Straightforward calculations show that ∂ mh (x) ⊤ ⊤ ∂Wx = = e⊤ (Xx Wx Xx )−1 Xx (Y − Xx α) 1 ∂hj ∂hj (8) ⊤ ⊤ where α = (Xx Wx Xx )−1 Xx Wx Y is the coefficient vector for the local linear fit. [sent-120, score-0.324]

52 Assuming a product kernel we have   d Wx = diag  j=1 d K((X1j − xj )/hj ), . [sent-122, score-0.143]

53 , j=1 K((Xnj − xj )/hj ) and ∂Wx /∂hj = Wx Dj where ∂ log K((X1j − xj )/hj ) ∂ log K((Xnj − xj )/hj ) Dj = diag ,. [sent-125, score-0.32]

54 For example, with the Gaussian 1 1 2 kernel K(u) = exp(−u /2) we have Dj = h3 diag (X1j − xj )2 , . [sent-129, score-0.143]

55 (12) i=1 Then the hard thresholding version of the rodeo algorithm is given in Figure 2. [sent-140, score-0.611]

56 Then E(σ 2 ) = σ 2 + bias where Rodeo: Hard thresholding version 1. [sent-146, score-0.154]

57 Select parameter 0 < β < 1 and initial bandwidth h0 slowly decreasing to zero, √ with h0 = Ω 1/ log log n . [sent-147, score-0.372]

58 Let cn = Ω(1) be a sequence satisfying dcn = Ω(log n). [sent-148, score-0.071]

59 Initialize the bandwidths, and activate all covariates: (a) hj = h0 , j = 1, 2, . [sent-150, score-0.367]

60 (c) If |Zj | ≥ λj , then set hj ← βhj , otherwise remove j from A. [sent-160, score-0.367]

61 Figure 2: The hard thresholding version of the rodeo, which can be applied using the derivatives Zj of any nonparametric smoother. [sent-166, score-0.258]

62 Note however that the bias is mitigated by sparsity (small r). [sent-169, score-0.069]

63 For convenience of notation we assume that the covariates are numbered such that the relevant variables xj correspond to 1 ≤ j ≤ r, and the irrelevant variables to j > r. [sent-173, score-0.354]

64 Suppose that K is a product kernel with bandwidth vector h = (h1 , . [sent-177, score-0.327]

65 More generally, assuming that r is bounded, we have the following when hj → 0: If j ∈ Rc the derivative of the bias is ∂ 2 2 µj = E[mH (x) − m(x)] = −tr (HR HR ) ν2 (∇j log f (x)) hj + oP (hj ) (13) ∂hj where the Hessian of m(x) is H = HR 0 0 0 and HR = diag(h2 , . [sent-182, score-0.83]

66 Let C = σ 2 R(K) 4m(x) = hj ν2 mjj (x) + oP (hj ). [sent-188, score-0.509]

67 (15) These lemmas parallel the calculations of Ruppert and Wand (1994) except for the difference that the irrelevant variables have different leading terms in the expansions than relevant variables. [sent-194, score-0.164]

68 (A1) For some constant k > 0, each j > r satisfies ∇j log f (x) = O (A2) For each j ≤ r, logk n n1/4 mjj (x) = 0 . [sent-197, score-0.209]

69 1 that Aj hj + oP (hj ) j ≤ r µj = (18) Bj hj + oP (hj ) j > r where 2 Aj = ν2 mjj (x), Bj = −tr(HH)ν2 (∇j log f (x))2 . [sent-200, score-0.903]

70 More generally, µj is approximately proportional to (∇j log f (x))2 for j > r which implies that |µj | ≈ 0 for irrelevant variables if f is sufficiently smooth in the variable xj . [sent-204, score-0.243]

71 Hence, assumption (A1) can be interpreted as requiring that f is sufficiently smooth in the irrelevant dimensions. [sent-205, score-0.068]

72 Equation (18) ensures that µj is proportional to hj |mjj (x)| for small hj . [sent-207, score-0.734]

73 Since we take the initial bandwidth h0 to be decreasingly slowly with n, (A2) implies that |µj (h)| ≥ chj |mjj (x)| for some constant c > 0, for sufficiently large n. [sent-208, score-0.296]

74 In the following we write Yn = OP (an ) to mean that Yn = OP (bn an ) where bn is logarithmic in n; similarly, an = Ω(bn ) if an = Ω(bn cn ) where cn is logarithmic in n. [sent-209, score-0.201]

75 Then the number of iterations Tn until the rodeo stops satisfies 1 1 P log1/β (nan ) ≤ Tn ≤ log1/β (nbn ) −→ 1 (20) 4+r 4+r where an = Ω(1) and bn = O(1). [sent-214, score-0.502]

76 Moreover, the algorithm outputs bandwidths h⋆ that satisfy 1 P h⋆ ≥ for all j > r −→ 1 (21) j logk n and P h0 (nbn )−1/(4+r) ≤ h⋆ ≤ h0 (nan )−1/(4+r) for all j ≤ r −→ 1 . [sent-215, score-0.287]

77 3, the risk R(h⋆ ) of the rodeo estimator satisfies R(h⋆ ) = OP n−4/(4+r) . [sent-219, score-0.593]

78 (23) In the one-dimensional case, this result shows that the algorithm recovers the locally optimal bandwidth, giving an adaptive estimator, and in general attains the optimal (up to logarithmic factors) minimax rate of convergence. [sent-220, score-0.126]

79 5 Some Examples and Extensions Figure 3 illustrates the rodeo on synthetic and real data. [sent-222, score-0.478]

80 The left plot shows the bandwidths obtained on a synthetic dataset with n = 500 points of dimension d = 20. [sent-223, score-0.28]

81 The covariates are generated as xi ∼ Uniform(0, 1), the true function is m(x) = 2(x1 +1)2 +2 sin(10x2 ), and σ = 1. [sent-224, score-0.155]

82 The results are averaged over 50 randomly generated data sets; note that the displayed bandwidth paths are not monotonic because of this averaging. [sent-225, score-0.329]

83 The plot shows how the bandwidths of the relevant variables shrink toward zero, while the bandwidths of the irrelevant variables remain large. [sent-226, score-0.665]

84 While we have focused on estimation of m locally at a point x, the idea can be extended to carry out global bandwidth and variable selection by averaging over multiple evaluation points x1 , . [sent-229, score-0.444]

85 In addition, it is possible to consider more general paths, for example using soft thresholding or changing only the bandwidth corresponding to the largest |Zj |/λj . [sent-234, score-0.449]

86 Such a version of the rodeo can be seen as a nonparametric counterpart to least angle regression (LARS) (Efron et al. [sent-235, score-0.662]

87 2004), a refinement of forward stagewise regression in which one adds the covariate most correlated with the residuals of the current fit, in small, incremental steps. [sent-236, score-0.103]

88 Reducing the bandwidth is like adding in more of that variable. [sent-238, score-0.296]

89 Suppose now that we make the following modifications to the rodeo: ∗ (i) change the bandwidths one at a time, based on the largest Zj = |Zj |/λj , (ii) reduce ∗ the bandwidth continuously, rather than in discrete steps, until the largest Zj is equal to the next largest. [sent-239, score-0.619]

90 Figure 3 (right) shows the result of running this greedy version of the rodeo on ∗ the diabetes dataset used to illustrate LARS. [sent-240, score-0.578]

91 The resulting variable ordering is seen to be very similar to, but different from, the ordering obtained from the parametric LARS fit. [sent-242, score-0.062]

92 0 12 0 20 40 60 80 100 Greedy Rodeo Step Figure 3: Left: Average bandwidth output by the rodeo for a function with r = 2 relevant variables in d = 20 dimensions (n = 500, with 50 trials). [sent-272, score-0.812]

93 Covariates are generated as xi ∼ Uniform(0, 1), the true function is m(x) = 2(x1 + 1)3 + 2 sin(10x2 ), and σ = 1, fit at the test point x = ( 1 , . [sent-273, score-0.049]

94 The variance is greater for large step sizes since the rodeo runs that long for fewer data 2 2 sets. [sent-277, score-0.445]

95 Right: Greedy rodeo on the diabetes data, used to illustrate LARS (Efron et al. [sent-278, score-0.523]

96 A set of k = 100 of the total n = 442 points were sampled (d = 10), and the bandwidth for the variable with largest average |Zj |/λj was reduced in each step. [sent-280, score-0.357]

97 The parametric LARS algorithm adds variables in the order 3, 9, 4, 7, 2, 10, 5, 8, 6, 1. [sent-282, score-0.079]

98 Optimal spatial adaptation to inhomogeneous smoothness: An approach based on kernel estimates with variable bandwidth selectors. [sent-334, score-0.366]

99 Empirical-bias bandwidths for local polynomial nonparametric regression and density estimation. [sent-354, score-0.388]

100 Variable selection and model building via likelihood basis pursuit. [sent-374, score-0.056]


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