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Local or Global Smoothing? A Bandwidth Selector for Dependent Data Francesco Giordano Maria Lucia Parrella Department of Economics and Statistics University of Salerno COMPSTAT 2010 F. Giordano M.L. Parrella (UNISA) Local or Global


  1. The plug-in optimal local bandwidth It is derived by minimizing the asymptotic mean square error h opt σ 2 ( x ; h ) } L ( x ) = arg min h AMSE { ˆ � 1 / ( 2 p + 3 ) � C p , K × v ( x ) = n × d 2 ( x ) × f X ( x ) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 26 / 115

  2. The plug-in optimal local bandwidth It is derived by minimizing the asymptotic mean square error h opt σ 2 ( x ; h ) } L ( x ) = arg min h AMSE { ˆ � 1 / ( 2 p + 3 ) � C p , K × v ( x ) = n × d 2 ( x ) × f X ( x ) The only unknown components (to estimate and plug-in) are ◮ v ( x ) , the conditional variance of X 2 t ; ◮ d ( x ) , the ( p + 1 ) -th derivative of σ 2 ( x ) ; ◮ f X ( x ) , the design density. F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 26 / 115

  3. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 27 / 115

  4. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 28 / 115

  5. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 29 / 115

  6. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 30 / 115

  7. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 31 / 115

  8. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 32 / 115

  9. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 33 / 115

  10. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 34 / 115

  11. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 35 / 115

  12. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 36 / 115

  13. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 37 / 115

  14. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 38 / 115

  15. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 39 / 115

  16. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 40 / 115

  17. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 41 / 115

  18. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 42 / 115

  19. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 43 / 115

  20. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 44 / 115

  21. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 45 / 115

  22. An illustrative example: local (variable) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 46 / 115

  23. The plug-in optimal global bandwidth It is derived by minimizing the integrated AMSE � h opt σ 2 ( x ; h ) } f X ( x ) dx = arg min AMSE { ˆ G h � 1 / ( 2 p + 3 ) � C p , K × R var = n × R f , bias F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 47 / 115

  24. The plug-in optimal global bandwidth It is derived by minimizing the integrated AMSE � h opt σ 2 ( x ; h ) } f X ( x ) dx = arg min AMSE { ˆ G h � 1 / ( 2 p + 3 ) � C p , K × R var = n × R f , bias The only unknown components (to estimate and plug-in) are: ◮ R var = � v ( x ) dx ; ◮ R f , bias = d 2 ( x ) f X ( x ) dx . � F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 47 / 115

  25. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 48 / 115

  26. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 49 / 115

  27. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 50 / 115

  28. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 51 / 115

  29. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 52 / 115

  30. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 53 / 115

  31. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 54 / 115

  32. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 55 / 115

  33. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 56 / 115

  34. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 57 / 115

  35. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 58 / 115

  36. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 59 / 115

  37. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 60 / 115

  38. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 61 / 115

  39. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 62 / 115

  40. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 63 / 115

  41. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x x+h X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 64 / 115

  42. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 65 / 115

  43. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 66 / 115

  44. An illustrative example: global (fixed) bandwidth true regression function LP estimate Kernel weights phi(X) x-h x X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 67 / 115

  45. Local vs global bandwidths The following relation holds: � � � � x ; h opt x ; h opt AMSE L ( x ) ≤ AMSE . G F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 68 / 115

  46. Local vs global bandwidths The following relation holds: � � � � x ; h opt x ; h opt AMSE L ( x ) ≤ AMSE . G Anyway, the estimation of h opt L ( x ) (=function) is less efficient than the estimation of h opt (=mean value) G F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 68 / 115

  47. Local vs global bandwidths The following relation holds: � � � � x ; h opt x ; h opt AMSE L ( x ) ≤ AMSE . G Anyway, the estimation of h opt L ( x ) (=function) is less efficient than the estimation of h opt (=mean value) G Some questions follow : ◮ what is the effective gain in using the local bandwidth? ◮ Is it always convenient to perform a local smoothing instead of a global smoothing? ◮ How to deal with pilot bandwidths? F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 68 / 115

  48. Our proposal: a two stage procedure Suppose we want to estimate the function σ 2 ( x ) on a support I X ⊂ R . F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 69 / 115

  49. Our proposal: a two stage procedure Suppose we want to estimate the function σ 2 ( x ) on a support I X ⊂ R . First stage: evaluating the “homogeneity” on the support 1 Given the estimates of h opt L ( x ) and h opt G , use some relative indicator in order to evaluate the potential gain in using the local bandwidth instead of the global bandwidth on I X . F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 69 / 115

  50. Our proposal: a two stage procedure Suppose we want to estimate the function σ 2 ( x ) on a support I X ⊂ R . First stage: evaluating the “homogeneity” on the support 1 Given the estimates of h opt L ( x ) and h opt G , use some relative indicator in order to evaluate the potential gain in using the local bandwidth instead of the global bandwidth on I X . Second stage: deriving the locally global bandwidth 2 Derive the optimal global bandwidths h opt on “homogeneous” G subsets of I X . F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 69 / 115

  51. Our proposal: a two stage procedure Suppose we want to estimate the function σ 2 ( x ) on a support I X ⊂ R . First stage: evaluating the “homogeneity” on the support 1 Given the estimates of h opt L ( x ) and h opt G , use some relative indicator in order to evaluate the potential gain in using the local bandwidth instead of the global bandwidth on I X . Second stage: deriving the locally global bandwidth 2 Derive the optimal global bandwidths h opt on “homogeneous” G subsets of I X . ◮ How to estimate h opt and h opt L ( x ) on I X ? G ◮ Which relative indicator to use? ◮ How to smooth such bandwidths on I X ? F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 69 / 115

  52. First stage : deriving the relative indicator Consider the relative increment of the AMSE, ∀ x ∈ I X , σ 2 ( x ; h opt σ 2 ( x ; h opt ∆ AMSE ( x ) = AMSE { ˆ G ) } − AMSE { ˆ L ( x )) } . σ 2 ( x ; h opt AMSE { ˆ L ( x )) } F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 70 / 115

  53. First stage : deriving the relative indicator Consider the relative increment of the AMSE, ∀ x ∈ I X , σ 2 ( x ; h opt σ 2 ( x ; h opt ∆ AMSE ( x ) = AMSE { ˆ G ) } − AMSE { ˆ L ( x )) } . σ 2 ( x ; h opt AMSE { ˆ L ( x )) } We managed to express the ∆ AMSE ( x ) of the estimator in a model free form: 2 p + 3 [ π h ( x )] − 2 ( p + 1 ) + 2 p + 2 1 ∆ AMSE ( π h ( x )) = 2 p + 3 [ π h ( x )] − 1 where π h ( x ) = h opt L ( x ) ≥ 0. h opt G F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 70 / 115

  54. First stage : deriving the relative indicator 2 p + 3 [ π h ( x )] − 2 ( p + 1 ) + 2 p + 2 1 ∆ AMSE ( π h ( x )) = 2 p + 3 [ π h ( x )] − 1 2.0 p=0 p=1 p=2 1.5 Delta_AMSE Delta_AMSE 1.0 0.5 beta 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 a 1 b pi(x) pi(x) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 71 / 115

  55. First stage : deriving the relative indicator 2 p + 3 [ π h ( x )] − 2 ( p + 1 ) + 2 p + 2 1 ∆ AMSE ( π h ( x )) = 2 p + 3 [ π h ( x )] − 1 2.0 p=0 p=1 p=2 1.5 Delta_AMSE Delta_AMSE 1.0 0.5 beta 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 a 1 b pi(x) pi(x) Remark 1 : as a function of π h ( x ) , the relative indicator ∆ AMSE ( x ) is completely model free. Remark 2 : ∆ AMSE < β iif π h ( x ) ∈ [ a β , b β ] , ∀ β > 0. F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 71 / 115

  56. A global measure of ∆ AMSE on the interval I X A global measure of ∆ AMSE would indicate, eventually, the need to use on the interval I X a local bandwidth. For example, the mean value of ∆ AMSE on the interval I X could be considered... � ∆ AMSE ( x ) f X ( x ) dx . I X ... but we get some advantages if we consider the median value of ∆ AMSE on the interval I X , or some other quantile. F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 72 / 115

  57. The median of ∆ AMSE on the interval I X Delta_AMSE * * * * beta o o o o o o o o o o o o o o o o o a 1 b pi(x) For a fixed threshold β , consider the two set of points S β = { x : x ∈ I X , π ( x ) ∈ [ a β , b β ] } ; S β = { x : x ∈ I X , π ( x ) / ∈ [ a β , b β ] Given the measure of the process µ X , the median of ∆ AMSE is the threshold value β ∗ for which µ X ( S β ∗ ) = µ X ( S β ∗ ) . F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 73 / 115

  58. The algorithm of the first stage Consider an estimation of both the global bandwidth h G and the 1 local bandwidth function h L ( X t ) , such as ◮ the neural network method proposed in the second stage; ◮ some other local method proposed in the literature. Estimate π h ( X t ) , for all X t ∈ I X , then search for β ∗ such that 2 � � I { ˆ π h ( X t ) ∈ [ a β ∗ , b β ∗ ] } ≈ I { ˆ π h ( X t ) / ∈ [ a β ∗ , b β ∗ ] } . X t ∈ I X X t ∈ I X where I ( · ) is the indicator function. Split the subset I X if β ∗ > τ , where τ represents the max relative 3 increment tolerated for ∆ AMSE when using a global bandwidth. F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 74 / 115

  59. Second stage : the locally global bandwidth Given the “homogeneous” subset I X ⊂ R , we want to estimate � 1 / ( 2 p + 3 ) � C p , K × R I X h opt var = G n × R I X f , bias where: ◮ R I X � var = I X v ( x ) d ω I X ; ◮ R I X � I X d 2 ( x ) f X ( x ) d ω I X . f , bias = dx How to derive the measure ω I X ? (we used d ω I X = µ ( I X ) ) 1 How to estimate v ( x ) and d 2 ( x ) on I X ? 2 F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 75 / 115

  60. Estimating the functions v ( x ) and d 2 ( x ) Note that: Var { X 2 t | X t − 1 = x } ≡ m 4 ( x ) − m 2 v ( x ) = 2 ( x ) ∂ p + 1 σ 2 ( x ) ≡ m ( p + 1 ) d ( x ) = ( x ) ∂ x p + 1 2 F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 76 / 115

  61. Estimating the functions v ( x ) and d 2 ( x ) Note that: Var { X 2 t | X t − 1 = x } ≡ m 4 ( x ) − m 2 v ( x ) = 2 ( x ) ∂ p + 1 σ 2 ( x ) ≡ m ( p + 1 ) d ( x ) = ( x ) ∂ x p + 1 2 As usual, we should have to estimate separately the three functions m ( p + 1 ) m 4 ( x ) , m 2 ( x ) , ( x ) 2 (Fan & Yao, 1998; Franke & Diagne, 2006) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 76 / 115

  62. Estimating the functions v ( x ) and d 2 ( x ) Note that: Var { X 2 t | X t − 1 = x } ≡ m 4 ( x ) − m 2 v ( x ) = 2 ( x ) ∂ p + 1 σ 2 ( x ) ≡ m ( p + 1 ) d ( x ) = ( x ) ∂ x p + 1 2 As usual, we should have to estimate separately the three functions m ( p + 1 ) m 4 ( x ) , m 2 ( x ) , ( x ) 2 (Fan & Yao, 1998; Franke & Diagne, 2006) An interesting result here is that we estimate the three functions by only one estimator! F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 76 / 115

  63. Estimating the functions v ( x ) and d 2 ( x ) We use only the following neural network estimator n � 2 � � X 2 η = arg min t − q ( X t − 1 ; η ) ˆ η t = 2 F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 77 / 115

  64. Estimating the functions v ( x ) and d 2 ( x ) We use only the following neural network estimator n � 2 � � X 2 η = arg min t − q ( X t − 1 ; η ) ˆ η t = 2 where ◮ q ( X t − 1 ; η ) = � d k = 1 c k Γ ( a k X t − 1 + b k ) + c 0 is the neural network function; ◮ η = ( c 0 , c 1 , . . . , c d , a 1 , . . . , a d , b 1 . . . b d ) is the vector of parameters to be estimated; ◮ d is the number of nodes in the hidden layer; ◮ Γ ( · ) is the logistic activation function . F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 77 / 115

  65. Estimating the functions v ( x ) and d 2 ( x ) By defining m 4 ε = E ( ǫ 4 t ) , we reparameterize as follows: m 4 ( x ) − m 2 2 ( x ) = m 2 v ( x ) = 2 ( x ) [ m 4 ε − 1 ] m ( p + 1 ) d ( x ) = ( x ) 2 then we propose the following estimators ˆ m 2 ( x ) = q ( x , ˆ η ) � n t = 2 X 4 t ˆ m 4 ε = � n η )] 2 t = 2 [ q ( X t , ˆ m ( p + 1 ) q ( p + 1 ) ( x ; ˆ ˆ ( x ) = η ) 2 F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 78 / 115

  66. Estimating the locally global bandwidth Finally, given the ergodicity of the process, the locally global bandwidth is estimated by: � 2 � m ( p + 1 ) ˆ � ( X t ) � n ∗ i = 1 ˆ v ( x i ) X t ∈ I X 2 ˆ R I X ˆ R I X var = . f , bias = , � n n t = 1 I ( X t ∈ I X ) � 1 / ( 2 p + 3 ) � C p , K × ˆ R I X ˆ var h I X = n × ˆ R I X f , bias The points { x 1 , x 2 , . . . , x n ∗ } in ˆ R I X var are uniformly spaced in the interval I X , and I ( · ) is the indicator function. F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 79 / 115

  67. Assumptions (a1) the errors ε t have continuous and positive density function and, for some δ > 4, E | ε t | δ < ∞ ; E ( ε 2 E ( ε t ) = E ( ε 3 t ) = 1 , t ) = 0 , (a2) the functions m ( · ) and σ ( · ) have continuous second derivative. Moreover, the function σ ( · ) is positive; (a3) there exist the constants M 1 > 0 and M 2 > 0 such that, for all y ∈ R , E | ε t | δ � 1 /δ � | m ( y ) | ≤ M 1 ( 1 + | y | ) , | σ ( y ) | ≤ M 2 ( 1 + | y | ) , M 1 + M 2 < 1 ; (a4) the density function f X ( · ) of the (stationary) measure of the process µ X exists, it is bounded, continuous and positive on every compact set in R . (b1) � d k = 1 | c k | ≤ ∆ n . (b2) d ≡ d n , d n → ∞ , ∆ n → ∞ as n → ∞ . ∆ 2 n d n log ( ∆ 2 n d n ) ∆ 4 (b3) Let K 1 ( n ) := and K 2 ( n ) := n 1 − δ . n √ n F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 80 / 115

  68. The consistency of the bandwidth selector Using the framework in Franke and Diagne (2006), we can state the following results. Lemma 1 Under assumptions (a1)-(a4) and (b1)-(b3), the volatility function estimator is consistent in the sense that: If K 1 ( n ) → 0 as n → ∞ , then � � � 2 η ) − σ 2 ( x ) E q ( x ; ˆ d µ X ( x ) → 0 n → ∞ if, additionally, K 2 ( n ) → 0 for some δ > 0, then � � � 2 a . s . η ) − σ 2 ( x ) q ( x ; ˆ d µ X ( x ) − → 0 n → ∞ F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 81 / 115

  69. The consistency of the bandwidth selector Lemma 2 Under assumptions (a1)-(a4) and (b1)-(b3), the estimator of the second derivative of σ 2 ( x ) is consistent in the sense that: If K 1 ( n ) → 0 as n → ∞ , then � � η ) − [ σ 2 ( x )] ′′ � 2 q ′′ ( x ; ˆ E d µ X ( x ) → 0 n → ∞ if, additionally, K 2 ( n ) → 0 for some δ > 0, then � � η ) − [ σ 2 ( x )] ′′ � 2 q ′′ ( x ; ˆ a . s . d µ X ( x ) − → 0 n → ∞ F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 82 / 115

  70. The consistency of the bandwidth selector Theorem 1 Under the conditions (a1)-(a4) and (b1)-(b3), ˆ R I X f , bias , with I X ⊆ R , is consistent in the sense that: If K 1 ( n ) → 0 as n → ∞ , then p R I X ˆ → R I X − n → ∞ f , bias f , bias if, additionally, K 2 ( n ) → 0 for some δ > 0, then ˆ a . s . R I X → R I X − n → ∞ f , bias f , bias Corollary Using the same conditions as in theorem 1, then ˆ m 4 ε is consistent in the sense that: p If K 1 ( n ) → 0 as n → ∞ , then ˆ m 4 ε − → m 4 ε n → ∞ a . s . if, additionally, K 2 ( n ) → 0 for some δ > 0, then ˆ m 4 ε − → m 4 ε n → ∞ F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 83 / 115

  71. The consistency of the bandwidth selector Theorem 2 Using the same conditions as in theorem 1, then ˆ R I X var with I X ⊂ R and n ∗ = O ( n ) , is consistent in the sense that: If K 1 ( n ) → 0 as n → ∞ , then p R I X ˆ → R I X − n → ∞ var var if, additionally, K 2 ( n ) → 0 for some δ > 0, then R I X ˆ a . s . → R I X − n → ∞ var var F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 84 / 115

  72. The consistency of the bandwidth selector For a fixed z ∈ R , let I z be a non null measure set which contains the point z . Let n z be the number of observed values in I z such that n z → ∞ when n → ∞ . Theorem 3 Using the same conditions as in Lemma 2, if n z = o ( n ) , for a set I X ⊂ R , then ˆ h I z is consistent in the sense that: If K 1 ( n ) → 0 as n → ∞ , then � � p � ˆ h I z − h opt sup L ( z ) − → 0 n → ∞ � � � z ∈ I X if, additionally, K 2 ( n ) → 0 for some δ > 0, then � � a . s . � ˆ h I z − h opt sup L ( z ) − → 0 n → ∞ � � � z ∈ I X F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 85 / 115

  73. The consistency of the bandwidth selector The previous results imply that: 1 p a . s . ˆ → h opt ˆ → h opt h G − h G − G . or G For all z ∈ I X 2 p a . s . ˆ → h opt ˆ → h opt h I z − L ( z ) h I z − L ( z ) , or Uniformly for each z ∈ I X 3 p a . s . π h ( z ) ˆ − → π h ( z ) ˆ π h ( z ) − → π h ( z ) or F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 86 / 115

  74. The simulation study: setting the models Model 1: X t = [ ψ ( X t − 1 + 1 . 2 ) + 1 . 5 ψ ( X t − 1 − 1 . 2 )] ǫ t ǫ t ∼ N ( 0 , 1 ) ψ ( z ) = d.f. of N ( 0 , 1 ) � 0 . 1 + 0 . 3 X 2 Model 2: X t = t − 1 ǫ t ǫ t ∼ N ( 0 , 1 ) � 0 . 01 + 0 . 1 X 2 t − 1 + 0 . 2 X 2 Model 3: X t = t − 1 I X t − 1 < 0 ǫ t ǫ t ∼ N ( 0 , 1 ) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 87 / 115

  75. Model 1: from Härdle and Tsybakov (1997) X t = [ ψ ( X t − 1 + 1 . 2 ) + 1 . 5 ψ ( X t − 1 − 1 . 2 )] ǫ t ; ǫ t ∼ N ( 0 , 1 ); ψ ( z ) = d . f . of N ( 0 , 1 ) TIME PLOT VOLATILITY FUNCTION 2 0.35 1 volatility 0.30 0 0.25 -1 0.20 -2 0 200 400 600 800 1000 -1.0 -0.5 0.0 0.5 1.0 X(-1) F. Giordano – M.L. Parrella (UNISA) Local or Global Smoothing? COMPSTAT 2010 - Paris 88 / 115

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