Hierarchical Modeling for Univariate Spatial Data

Transcription

1 Univariate spatial models Spatial Domain Hierarchical Modeling for Univariate Spatial Data Sudipto Banerjee and Andrew O. Finley 2 Biostatistics, School of Public Health, University of Minnesota, Minneapolis, Minnesota, U.S.A. 2 Department of Forestry & Department of Geography, Michigan State University, Lansing Michigan, U.S.A. July 25, 204 y x 2 Graduate Workshop on Environmental Data Analytics 204 Univariate spatial models Algorithmic Modeling Univariate spatial models What is a spatial process? Spatial surface observed at finite set of locations S = {s, s 2,..., s n } Tessellate the spatial domain (usually with data locations as vertices) Fit an interpolating polynomial: x x f(s) = i w i (S ; s)f(s i ) Y(s) Y(s2) Interpolate by reading off f(s 0 ). Includes: triangulation, weighted averages, geographically weighted regression (GWR) Issues: Sensitivity to tessellations Choices of multivariate interpolators Numerical error analysis x Y(sn) x 3 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Univariate spatial models Simple linear model Univariate spatial models Simple linear model Simple linear model Simple linear model y(s) = µ(s) + ɛ(s), Response: y(s) at location s Mean: µ = x(s) β Error: ɛ(s) iid N(0, τ 2 ) Assumptions regarding ɛ(s): ɛ(s) iid N(0, τ 2 ) y(s) = µ(s) + ɛ(s), ɛ(s i ) and ɛ(s j ) are uncorrelated for all i j D D y(s), x(s) ɛ(s i) ɛ(s j) 5 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204

2 Univariate spatial models Sources of variation Univariate spatial models Sources of variation Spatial Gaussian processes (GP ): Say w(s) GP (0, σ 2 ρ( )) and Cov(w(s ), w(s 2 )) = σ 2 ρ (φ; s s 2 ) Let w = [w(s i )] n i=, then w N(0, σ 2 R(φ)), where R(φ) = [ρ(φ; s i s j )] n i,j= D Realization of a Gaussian process: Changing φ and holding σ 2 = : w N(0, σ 2 R(φ)), where R(φ) = [ρ(φ; s i s j )] n i,j= Correlation model for R(φ): e.g., exponential decay ρ(φ; t) = exp( φt) if t > 0. w(s i) w(s j) Other valid models e.g., Gaussian, Spherical, Matérn. Effective range, t 0 = ln(0.05)/φ 3/φ 7 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Univariate spatial models Sources of variation Univariate spatial models Univariate spatial regression w N(0, σ 2 wr(φ)) provides complex spatial dependence through simple structured dependence. E.g., anisotropic Matérn correlation function: ρ(s i, s j; φ) = ( /Γ(ν)2 ν ) ( 2 νd ij) ν κ ν(2 νd ij ), where d ij = (s i s j) Σ (s i s j), Σ = G(ψ)Λ 2 G(ψ). Thus, φ = (ν, ψ, Λ). Simple linear model + random spatial effects y(s) = µ(s) + w(s) + ɛ(s), Simulated Predicted Response: y(s) at some site Mean: µ = x(s) β Spatial random effects: w(s) GP (0, σ 2 ρ(φ; s s 2 )) Non-spatial variance: ɛ(s) iid N(0, τ 2 ). Interpretation as pure error, measurement error, replication error, microscale error. 9 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Univariate spatial models Univariate spatial regression Univariate spatial models Univariate spatial regression First stage: y β, w, τ 2 Second stage: Hierarchical modeling n N(y(s i ) x(s i ) β + w(s i ), τ 2 ) i= w σ 2, φ N(0, σ 2 R(φ)) Third stage: Priors on Ω = (β, τ 2, σ 2, φ) Marginalized likelihood: y Ω N(Xβ, σ 2 R(φ) + τ 2 I) Note: Spatial process parametrizes Σ: y = Xβ + ɛ, ɛ N (0, Σ), Σ = σ 2 R(φ) + τ 2 I Graduate Workshop on Environmental Data Analytics 204 Bayesian Computations Choice: Fit [y Ω] [Ω] or [y β, w, τ 2 ] [w σ 2, φ] [Ω]. Conditional model: conjugate full conditionals for β, σ 2, τ 2 and w easier to program. Marginalized model: need Metropolis or Slice sampling for σ 2, τ 2 and φ. Harder to program. But, reduced parameter space faster convergence σ 2 R(φ) + τ 2 I is more stable than σ 2 R(φ). But what about R (φ)?? EXPENSIVE! 2 Graduate Workshop on Environmental Data Analytics 204

3 Univariate spatial models Univariate spatial regression Univariate spatial models Spatial Prediction Where are the w s? Often we need to predict y(s) at a new set of locations {s 0,..., s m } with associated predictor matrix X. Interest often lies in the spatial surface w y. Sample from predictive distribution: Z [y y, X, X ] = [y, Ω y, X, X ]dω Z = [y y, Ω, X, X ] [Ω y, X]dΩ, They are recovered from Z [w y, X] = [w Ω, y, X] [Ω y, X]dΩ using posterior samples: Obtain Ω(),..., Ω(G) [Ω y, X] For each Ω(g), draw w(g) [w Ω(g), y, X] [y y, Ω, X, X ] is multivariate normal. Sampling scheme: Obtain Ω(),..., Ω(G) [Ω y, X] (g) For each Ω(g), draw y [y y, Ω(g), X, X ]. NOTE: With Gaussian likelihoods [w Ω, y, X] is also Gaussian. With other likelihoods this may not be a standard distribution; conditional updating scheme is preferred. 3 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Illustration Illustration Colorado data illustration Colorado data illustration Modeling temperature: 50 locations in Colorado. Simple spatial regression model: y(s) = x(s)> β + w(s) + (s) iid w(s) GP (0, σ 2 ρ( ; φ, ν)); (s) N (0, τ 2 ) Parameters Intercept [Elevation] Precipitation σ2 φ Range τ2 5 Graduate Workshop on Environmental Data Analytics % (2.5%,97.5%) (2.3,3.866) (-0.527,-0.333) (0.002,0.072) 0.34 (0.05,.245) 7.39E-3 (4.7E-3, 5.2E-3) (38.8, 476.3) 0.05 (0.022, 0.092) 6 Graduate Workshop on Environmental Data Analytics 204 Illustration Illustration Elevation map 39 Latitude Northing Temperature residual map Easting 7 Graduate Workshop on Environmental Data Analytics Longitude 8 Graduate Workshop on Environmental Data Analytics 204

4 Illustration Residual map with elev. as covariate Northing Easting 9 Graduate Workshop on Environmental Data Analytics 204

5 Computing environments Brief notes on setting up semi-high performance computing environments July 25, 204 We have two different computing environments for fitting demanding models to large space and/or time data sets. A distributed system consists of multiple autonomous computers (nodes) that communicate through a co mputer network. A computer program that runs in a distributed system is called a distributed program. Message Passing Interface (MPI) is a specification for an Application Programming Interface (API) that allows many computers to communicate with one another (implementations in C, C++, and Fortran.) 2 A shared memory multiprocessing system consists of a single computer with memory that may be simultaneously accessed by one or more programs running on multiple central processing units (CPUs). The OpenMP (Open Multi-Processing) is an API that supports shared memory multiprocessing programming (implementations in C, C++, and Fortran). 2 Graduate Workshop on Environmental Data Analytics 204 Computing environments We have two different computing environments for fitting demanding models to large space and/or time data sets. Recent work focuses on fitting geostatistical (specifically point-referenced) models using MCMC methods. This necessitates iterative evaluation of a likelihood which requires operations on large matrices. A specific hurdle is factorization to computing determinant and inverse of large dense covariance matrices. We try to model our way out and use tools from computer science to overcome the computational complexity (e.g., covariance tapering, Kaufman et al. 2008; low-rank methods, Cressie and Johannesson 2008; Banerjee et al. 2008, etc.). Due to slow network communication and transport of submatrices among nodes distributed systems are not ideal for these types of iterative large matrix operations. 3 Graduate Workshop on Environmental Data Analytics 204 Computing environments My lab currently favors shared memory multiprocessing system. We buy rack mounted units (e.g., Sun Fire X470 Server with 2 quad-core Intel Xeon Processor 5500 Series and 48 GB of RAM 0-5k) running the Linux operating systems. Software includes OpenMP coupled with Intel Math Kernel Library (MKL) non-commercial-software-development. MKL is a library of highly optimized, extensively threaded math routines (e.g., BLAS, LAPACK, ScaLAPACK, Sparse Solvers, Fast Fourier Transforms, and vector RNGs). 4 Graduate Workshop on Environmental Data Analytics 204 Computing environments Computing environments So what kind of speed up to expect from threaded BLAS and LAPACK libraries. Mean computing times of dpotrf: See for some simple examples of C++ with MKL and Rmath libraries along with associated Makefile files (I ll add more examples shortly and upon request). 5 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204

6 Computing environments Computing environments Many core and contributed packages (including spbayes) call Basic Linear Algebra Subprograms (BLAS) and LAPACK (Linear Algebra PACKage) Fortran libraries. Substantial computing gains: processor specific threaded BLAS/LAPACK implementation (e.g., MKL or AMD s Core Math Library (ACML)) processor specific compilers (e.g., Intel s icc/ifort) Compiling R to call MKL s BLAS and LAPACK libraries (rather than stock serial versions). MKL_LIB_PATH="/opt/intel/composer_xe_20_sp.0.39/mkl/lib/intel64" export LD_LIBRARY_PATH=$MKL_LIB_PATH MKL="-L${MKL_LIB_PATH} -lmkl_intel_lp64 -lmkl_intel_thread \ -lmkl_core -liomp5 -lpthread -lm"./configure --with-blas="$mkl" --with-lapack 7 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Computing environments For many BLAS and LAPACK functions calls from R, expect near linear speed up... 9 Graduate Workshop on Environmental Data Analytics 204

7 Spatial Generalized Linear Models Hierarchical Modeling for non-gaussian Spatial Data Sudipto Banerjee and Andrew O. Finley 2 Biostatistics, School of Public Health, University of Minnesota, Minneapolis, Minnesota, U.S.A. 2 Department of Forestry & Department of Geography, Michigan State University, Lansing Michigan, U.S.A. Often data sets preclude Gaussian modeling: y(s) may not even be continuous Example: y(s) is a binary or count variable species presence or absence at location s species abundance from count at location s continuous forest variable is high or low at location s Replace Gaussian likelihood by exponential family member July 25, 204 Spatial GLM, Diggle Tawn and Moyeed (998) 2 Graduate Workshop on Environmental Data Analytics 204 Spatial Generalized Linear Models Spatial Generalized Linear Models Comments First stage: y(s i ) are conditionally independent given β and w(s i ), so f(y(s i ) β, w(s i ), γ) equals h(y(s i ), γ) exp (γ[y(s i )η(s i ) ψ(η(s i ))]) where g(e(y(s i ))) = η(s i ) = x (s i )β + w(s i ) (canonical link function) and γ is a dispersion parameter. Second stage: Model w(s) as a Gaussian process: w N(0, σ 2 R(φ)) Third stage: Priors and hyperpriors. No process for y(s), only a valid joint distribution Not sensible to add a pure error term ɛ(s) We are modeling with spatial random effects Introducing these in the transformed mean encourages means of spatial variables at proximate locations to be close to each other Marginal spatial dependence is induced between, say, y(s) and y(s ), but observed y(s) and y(s ) need not be close to each other Second stage spatial modeling is attractive for spatial explanation in the mean First stage spatial modeling more appropriate to encourage proximate observations to be similar. 3 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Illustration Illustration Binary spatial regression: forest/non-forest Illustration from: Finley, A.O., S. Banerjee, and R.E. McRoberts. (2008) A Bayesian approach to quantifying uncertainty in multi-source forest area estimates. Environmental and Ecological Statistics, 5: We illustrate a non-gaussian model for point-referenced spatial data: Objective is to make pixel-level prediction of forest/non-forest across the domain. Data: Observations are from 500 georeferenced USDA Forest Service Forest Inventory and Analysis (FIA) inventory plots within a 32 km radius circle in MN, USA. The response y(s) is a binary variable, with { if inventory plot is forested y(s) = 0 if inventory plot is not forested Observed covariates include the coinsiding pixel values for 3 dates of m resolution Landsat imagery. 5 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204

8 Illustration Illustration Posterior parameter estimates Binary spatial regression: forest/non-forest Parameter estimates (posterior medians and upper and lower 2.5 percentiles): We fit a generalized linear model where y(si ) Bernoulli(p(si )), logit(p(si )) = x(si )0 β + w(si ). Parameters Intercept (θ0 ) AprilTC (θ ) AprilTC2 (θ2 ) AprilTC3 (θ3 ) JulyTC (θ4 ) JulyTC2 (θ5 ) JulyTC3 (θ6 ) OctTC (θ7 ) OctTC2 (θ8 ) OctTC3 (θ9 ) σ2 φ log(0.05)/φ (meters) Assume vague flat for β, a Uniform(3/32, 3/0.5) prior for φ, and an inverse-gamma(2, ) prior for σ 2. Parameters updated with Metropolis algorithm using target log density: ln (p(ω y)) σb n σa + + ln σ 2 2 ln ( R(φ) ) 2 w0 R(φ) w 2 σ 2 2σ n n X X + y(si ) x(si )0 β + w(si ) ln + exp(x(si )0 β + w(si ) i= i= Covariates and proximity to observed FIA plot will contribute to increase precision of prediction. + ln(σ 2 ) + ln(φ φa ) + ln(φb φ). 7 Graduate Workshop on Environmental Data Analytics Illustration Graduate Workshop on Environmental Data Analytics 204 Illustration cut point F[P(Y(A) = )] CDF of holdout area s posterior predictive distributions P(Y(A) = ) Classification of pixel areas (based on visual inspection of imagery) into non-forest (), moderately forest ( ), and forest (no marker). Graduate Workshop on Environmental Data Analytics 204 Illustration Median of posterior predictive distributions 9 Estimates: 50% (2.5%, 97.5%) (49.56, 20.46) (-0.45, -0.) 0.7 (0.07, 0.29) (-0.43, -0.08) (-0.25, 0.7) 0.09 (-0.0, 0.9) 0.0 (-0.5, 0.6) (-0.68, -0.22) (-0.9, 0.4) (-0.46, -0.07).358 (0.39, 2.42) ( , ) (932.33, ) Graduate Workshop on Environmental Data Analytics %-2.5% range of posterior predictive distributions 0 Graduate Workshop on Environmental Data Analytics 204

9 Spatio-temporal Models Building simple spatiotemporal models Hierarchical Modeling for Spatialtemporal Data Sudipto Banerjee and Andrew O. Finley 2 Biostatistics, School of Public Health, University of Minnesota, Minneapolis, Minnesota, U.S.A. 2 Department of Forestry & Department of Geography, Michigan State University, Lansing Michigan, U.S.A. July 25, 204 Modeling: Y t (s) = µ t (s) + w t (s) + ɛ t (s), or perhaps g(e(y t (s)) = µ t (s) + w t (s) For ɛ t (s), independent N(0, τ 2 t ) For w t (s) w t (s) = α t + w(s) w t (s) independent for each t w t (s) = w t (s) + η t (s), independent spatial process innovations 2 Graduate Workshop on Environmental Data Analytics 204 Spatio-temporal Models Univariate dynamic spatiotemporal models Spatio-temporal Models Dynamic Predictive Process Models Measurement Equation Y (s, t) = µ(s, t) + ɛ(s, t); ɛ(s, t) ind N(0, σ 2 ɛ ). µ(s, t) = x(s, t) β(s, t). β(s, t) = β t + β(s, t) Transition Equation β t = β t + η t, η t ind N p (0, Ση) β(s, t) = β(s, t ) + w(s, t). w (s, t) = Av (s, t), with v (s, t) = (v (s, t),..., v p (s, t)). The v l (s, t) s are replications of a Gaussian processes with unit variance and correlation function ρ l (φ l, ) Connect to linear Kalman filter 3 Graduate Workshop on Environmental Data Analytics 204 Dynamic models for large spatiotemporal datasets Measurement Equation Y (s, t) = µ(s, t) + ɛ(s, t); ɛ(s, t) ind N(0, σ 2 ɛ ). µ(s, t) = x(s, t) β(s, t). β(s, t) = β t + β(s, t) Transition Equation β t = β t + η t, η t ind N p (0, Ση) β(s, t) = β(s, t ) + w(s, t). w (s, t) = Aṽ (s, t), where ṽ (s, t) = E[v(s, t) v ]. 4 Graduate Workshop on Environmental Data Analytics 204

10 Multivariate spatial modeling Modeling Multivaraite Spatial Data Sudipto Banerjee and Andrew O. Finley 2 Biostatistics, School of Public Health, University of Minnesota, Minneapolis, Minnesota, U.S.A. 2 Department of Forestry & Department of Geography, Michigan State University, Lansing Michigan, U.S.A. July 25, 204 Point-referenced spatial data often come as multivariate measurements at each location. Examples: Environmental monitoring: stations yield measurements on ozone, NO, CO, and PM 2.5. Community ecology: assemblages of plant species due to water availibility, temperature, and light requirements. Forestry: measurements of stand characteristics age, total biomass, and average tree diameter. Atmospheric modeling: at a given site we observe surface temperature, precipitation and wind speed We anticipate dependence between measurements at a particular location across locations 2 Graduate Workshop on Environmental Data Analytics 204 Multivariate spatial modeling Multivariate spatial modeling Bivariate Linear Spatial Regression A single covariate X(s) and a univariate response Y (s) At any arbitrary point in the domain, we conceive a linear spatial relationship: E[Y (s) X(s)] = β 0 + β X(s); where X(s) and Y (s) are spatial processes. Regression on uncountable sets: Regress {Y (s) : s D} on {X(s) : s D}. Inference: Estimate β 0 and β. Estimate spatial surface {X(s) : s D}. Estimate spatial surface {Y (s) : s D}. 3 Graduate Workshop on Environmental Data Analytics 204 Bivariate spatial process A bivariate distribution [Y, X] will yield regression [Y X]. So why not start with a bivariate process? [ ] ([ ] [ ]) X(s) µx (s) CXX ( ; θ Z(s) = GP Y (s) 2, Z ) C XY ( ; θ Z ) µ Y (s) C Y X ( ; θ Z ) C Y Y ( ; θ Z ) The cross-covariance function: [ ] CXX (s, t; θ C Z (s, t; θ Z ) = Z ) C XY (s, t; θ Z ), C Y X (s, t; θ Z ) C Y Y (s, t; θ Z ) where C XY (s, t) = cov(x(s), Y (t)) and so on. 4 Graduate Workshop on Environmental Data Analytics 204 Multivariate spatial modeling Multivariate spatial modeling Cross-covariance functions satisfy certain properties: C XY (s, t) = cov(x(s), Y (t)) = cov(y (t), X(s)) = C Y X (t, s). Caution: C XY (s, t) C XY (t, s) and C XY (s, t) C Y X (s, t). In matrix terms, C Z (s, t; θ Z ) = C Z (t, s; θ Z ) Positive-definiteness for any finite collection of points: n i= j= n a i C Z (s i, t j ; θ Z )a j > 0 for all a i R 2 \ {0}. Bivariare Spatial Regression from a Separable Process To ensure E[Y (s) X(s)] = β 0 + β X(s), we assume [ ] ([ ] [ ]) X(s) µ T T Z(s) = N, 2 for every s D Y (s) µ 2 T 2 T 22 Simplifying assumption : C Z (s, t) = ρ(s, t)t = Σ Z = {ρ(s i, s j )T} = R(φ) T. 5 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204

11 Multivariate spatial modeling Multivariate spatial modeling Then, p(y (s) X(s)) = N(Y (s) β 0 + β X(s), σ 2 ), where β 0 = µ 2 T 2 T µ, β = T 2 T, σ 2 = T 22 T 2 2 T. Regression coefficients are functions of process parameters. Estimate {µ, µ 2, T, T 2, T 22 } by sampling from p(φ) N(µ δ, V µ ) IW (T r, S) N(Z µ, R(φ) T) Immediately obtain posterior samples of {β 0, β, σ 2 }. Bivariate Spatial Regression with Misalignment Rearrange the components of Z to Z = (X(s ), X(s 2 ),..., X(s n ), Y (s ), Y (s 2 ),..., Y (s n )) yields [ ] X N Y ([ ] ) µ, T R (φ). µ 2 Priors: Wishart for T, normal (perhaps flat) for (µ, µ 2 ), discrete prior for φ or perhaps a uniform on (0,.5max dist). Estimation: Markov chain Monte Carlo (Gibbs, Metropolis, Slice, HMC/NUTS); Integrated Nested Laplace Approximation (INLA). 7 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Multivariate spatial modeling Multivariate spatial modeling Hierarchical approach (Royle and Berliner, 999; Cressie and Wikle, 20) Y (s) and X(s) observed over a finite set of locations S = {s, s 2,..., s n }. Y and X are n vectors of observed Y (s i ) s and X(s i ) s, respectively. How do we model Y X? No conditional process meaningless to talk about the joint distribution of Y (s i ) X(s i ) and Y (s j ) X(s j ) for two distinct locations s i and s j. Can model using [X] [Y X] but can we interpolate/predict at arbitrary locations? Hierarchical approach (contd.) X(s) GP (µ X (s), C X ( ; θ X )). Therefore, X N(µ X, C X (θ X )). C X (θ X ) is n n with entries C X (s i, s j ; θ X ). e(s) GP (0, C e ( ; θ e )); C e is analogous to C X. Y (s i ) = β 0 + β X(s i ) + e(s i ), for i =, 2,..., n. Joint distribution of Y and X: ( ) ([ ] [ ]) X µx CX (θ N, X ) β C X (θ X ) Y µ Y β C X (θ X ) C e (θ e ) + βc 2, X (θ X ) where µ Y = β 0 + β µ X. 9 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Multivariate spatial modeling Multivariate spatial modeling This joint distribution arises from a bivariate spatial process: [ ] [ ] X(s) µ W(s) = and E[W(s)] = µ Y (s) W (s) = X (s). β 0 + β µ X (s) and cross-covariance [ C W (s, s CX (s, s ) = ) β C X (s, s ] ) β C X (s, s ) β 2C X(s, s ) + C e (s, s, ) where we have suppressed the dependence of C X (s, s ) and C e (s, s ) on θ X and θ e respectively. This implies that E[Y (s) X(s)] = β 0 + β X(s) for any arbitrary location s, thereby specifying a well-defined spatial regression model for an arbitrary s. Coregionalization (Wackernagel) Separable models assume one spatial range for both X(s) and Y (s). Coregionalization helps to introduce a second range parameter. Introduce two latent independent GP s, each having its own parameters: v (s) GP (0, ρ ( ; φ )) and v 2 (s) GP (0, ρ 2 ( ; φ 2 )) Construct a bivariate process as the linear transformation: w (s) = a v (s) w 2 (s) = a 2 v (s) + a 22 v 2 (s) Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204

12 Multivariate spatial modeling Multivariate spatial modeling Short form: Coregionalization [ ] [ ] a 0 v (s) w(s) = a 2 a 22 v 2 (s) = Av(s) Cross-covariance of v(s): [ ] ρ (s, t; φ C v (s, t) = ) 0 0 ρ 2 (s, t; φ 2 ) Cross-covariance of w(s): C w (s, t) = AC v (s, t)a. It is a valid cross-covariance function (by construction). If s = t, then C w (s, s) = AA. No loss of generality to specify A as (lower) triangular. 3 Graduate Workshop on Environmental Data Analytics 204 If v (s) and v 2 (s) have identical correlation functions, then ρ (s, t) = ρ 2 (s, t) and C w (s) = ρ(s, t; φ)aa = separable model Coregionalized Spatial Linear Model [ ] X(s) = Y (s) [ ] µx (s) + µ Y (s) [ ] w (s) + w 2 (s) [ ] ex (s), e Y (s) where e X (s) and e Y (s) are independent white-noise processes [ ] ([ ] [ ]) ex (s) 0 τ 2 N e Y (s) 2, X τy 2 for every s D. 4 Graduate Workshop on Environmental Data Analytics 204 Multivariate spatial modeling Multivariate spatial modeling Generalizations Each location contains m spatial regressions Y k (s) = µ k (s) + w k (s) + ɛ k (s), k =,..., m. Let v k (s) GP (0, ρ k (s, s )), for k =,..., m be m independent GP s with unit variance. Assume w(s) = A(s)v(s) arises as a space-varying linear transformation of v(s). Then: C w (s, t) = A(s)C v (s, t)a (t) is a valid cross-covariance function. A(s) is unknown! Should we first model A(s) to obtain C w (s, s)? Or should we model C w (s, t) first and derive A(s)? A(s) is completely determined from within-site associations. 5 Graduate Workshop on Environmental Data Analytics 204 Other approaches for cross-covariance models Convolutions of processes and covariance functions Gaspari and Cohn (Quart. J. Roy. Met. Soc., 999). Majumdar and Gelfand (Math. Geo., 2007). Latent dimension approach: Apanasovich and Genton (Biometrika, 200). Apanasovich et al. (JASA, 202). Multivariate Matérn family Gneiting et al. (JASA, 200). Nonstationary variants of coregionalization Space-varying: Gelfand et al. (Test, 200). Dimension-reducing (over space): Guhaniyogi et al. (JABES, 202). Dimension-reducing (over outcomes): Ren and Banerjee (Biometrics, 203). Variogram modeling: De Iaco et al. (Math. Geo., 2003). 6 Graduate Workshop on Environmental Data Analytics 204

13 Hierarchical spatial process models Hierarchical Spatial model Modeling Large Spatial Datasets Sudipto Banerjee and Andrew O. Finley 2 Biostatistics, School of Public Health, University of Minnesota, Minneapolis, Minnesota, U.S.A. 2 Department of Forestry & Department of Geography, Michigan State University, Lansing Michigan, U.S.A. July 25, 204 p(θ) p(ψ) N(β µ β, Σ β ) N(w 0, C w ( θ )) n N m (y(s i ) X(s i ) β + w(s i ), D(Ψ) ) i= regression slopes spatial random effects from Gaussian process nonspatial variability (nugget) spatial process parameters (spatial variance, range, smoothness) and. 2 Graduate Workshop on Environmental Data Analytics 204 Computational issues Dimension reduction Some approaches Approaches to dimension reduction: We need to evaluate Computational issues. 2 log det(c w(θ)) 2 w C w (θ) w What if n is LARGE? How do we tackle C w (θ) and det(c(θ))? 3 Graduate Workshop on Environmental Data Analytics 204 Covariance tapering (Furrer et al. 2006; Zhang and Du, 2007; Du et al. 2009; Kaufman et al., 2009) Spectral domain: (Fuentes 2007; Paciorek, 2007) Approximate using GMRFs: INLA (Rue et al. 2009; Lindgren et al., 20) Nearest-neighbor models (processes) (Vecchia 988; Stein et al. 2004; Datta et al., 204) Low-rank approaches (Wahba, 990; Higdon, 2002; Lin et al., 2000; Kamman & Wand, 2003; Paciorek, 2007; Rasmussen & Williams, 2006; Stein 2007, 2008; Cressie & Johannesson, 2008; Banerjee et al., 2008; 200; Sang et al., 20) 4 Graduate Workshop on Environmental Data Analytics 204 Dimension reduction Some approaches Dimension reduction Some approaches Higdon (2002) proposed kernel convolution approximations. S = {s, s 2,..., s n}: a set of knots. w(s) w KC (s) = k(s s j, θ )u j, n j= Smoothing causes loss in variability: w(s) w KC (s) = u j iid N(0, ). k(s v, θ )du(v) k(s s j, θ )u j j=n + k(s s j, θ )u j, n j= No easy way to quantify this difference with kernel convolutions. 5 Graduate Workshop on Environmental Data Analytics 204 Low rank Gaussian process Call w(s) GP m (0, C θ ( )) the parent process For S = {s, s 2,..., s n }, let C w (θ) = { C θ (s i, s j ) } : w = (w(s ), w(s 2),..., w(s n ) ) N(0, C w(θ)) The predictive process derived from w(s) is: w(s) = E[w(s) w ] = cov{w(s), w } var{w } w. w(s) is a degenerate Gaussian process delivering dimension-reduction. 6 Graduate Workshop on Environmental Data Analytics 204

14 y x y x Hierarchical predictive process models w(s) Low rank interpolation w(s) = z(s, θ) w w = (w(s ),..., w(s n ) ) Hierarchical predictive process models tauˆ knots Parent process surface Predictive process surface Hierarchical predictive process models p(θ) p(ψ) N(β µ β, Σ β ) N(w 0, C w(θ)) n N m (y(s i ) X(s i ) β + w(s i ), D(Ψ)). i= Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Hierarchical predictive process models Systemic under-estimation: Systematic under-estimation var{w(s)} = var{e[w(s) w ]} + E{var[w(s) w ]} var{e[w(s) w ]} = var{ w(s)}. Orthogonal decomposition: var{w(s)} = var{ w(s)} + var{w(s) w(s)} ɛ(s) = w(s) w(s) GP (0, C ɛ (s, s 2 ; θ )): C ɛ (s, s 2 ; θ ) = C(s, s 2 ; θ ) c(s ; θ ) C (θ ) c(s 2 ; θ 2 ). 9 Graduate Workshop on Environmental Data Analytics 204

15 Introduction to Spatial Data and Models Sudipto Banerjee and Andrew O. Finley 2 Biostatistics, School of Public Health, University of Minnesota, Minneapolis, Minnesota, U.S.A. 2 Department of Forestry & Department of Geography, Michigan State University, Lansing Michigan, U.S.A. July 25, 204 Researchers in diverse areas such as climatology, ecology, environmental health, and real estate marketing are increasingly faced with the task of analyzing data that are: highly multivariate, with many important predictors and response variables, geographically referenced, and often presented as maps, and temporally correlated, as in longitudinal or other time series structures. motivates hierarchical modeling and data analysis for complex spatial (and spatiotemporal) data sets. 2 Graduate Workshop on Environmental Data Analytics 204 Type of spatial data Exploration of spatial data point-referenced data, where y(s) is a random vector at a location s R r, where s varies continuously over D, a fixed subset of R r that contains an r-dimensional rectangle of positive volume; areal data, where D is again a fixed subset (of regular or irregular shape), but now partitioned into a finite number of areal units with well-defined boundaries; point pattern data, where now D is itself random; its index set gives the locations of random events that are the spatial point pattern. y(s) itself can simply equal for all s D (indicating occurrence of the event), or possibly give some additional covariate information (producing a marked point pattern process). First step in analyzing data First Law of Geography: Mean + Error Mean: first-order behavior Error: second-order behavior (covariance function) EDA tools examine both first and second order behavior Preliminary displays: Simple locations to surface displays 3 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Exploration of spatial data Exploration of spatial data First Law of Geography Scallops Sites data = mean + error 5 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204

16 Deterministic surface interpolation Spatial surface observed at finite set of locations S = {s, s 2,..., s n } Tessellate the spatial domain (usually with data locations as vertices) Fit an interpolating polynomial: f(s) = i w i (S ; s)f(s i ) Interpolate by reading off f(s 0 ). Issues: Sensitivity to tessellations Choices of multivariate interpolators Numerical error analysis 7 Graduate Workshop on Environmental Data Analytics 204 Scallops data: image and contour plots Longitude Latitude 8 Graduate Workshop on Environmental Data Analytics 204 Scallops data: image and contour plots Drop-line scatter plot 9 Graduate Workshop on Environmental Data Analytics 204 Scallops data: image and contour plots Surface plot Longitude Latitude logsp 0 Graduate Workshop on Environmental Data Analytics 204 Scallops data: image and contour plots Image contour plot Longitude Latitude Graduate Workshop on Environmental Data Analytics 204 Scallops data: image and contour plots Locations form patterns Eastings Northings Graduate Workshop on Environmental Data Analytics 204

17 Scallops data: image and contour plots Scallops data: image and contour plots Surface features Interesting plot arrangements Shrub Density Northings Eastings N S UTM coordinates E W UTM coordinates 3 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Elements of point-level modeling Stationary Gaussian processes Point-level modeling refers to modeling of spatial data collected at locations referenced by coordinates (e.g., lat-long, Easting-Northing). Fundamental concept: Data from a spatial process {y(s) : s D}, where D is a fixed subset in Euclidean space. Example: y(s) is a pollutant level at site s Conceptually: Pollutant level exists at all possible sites Practically: Data will be a partial realization of a spatial process observed at {s,..., s n } Statistical objectives: Inference about the process y(s); predict at new locations. Suppose our spatial process has a mean, µ (s) = E (y (s)), and that the variance of y(s) exists for all s D. Strong stationarity: If for any given set of sites, and any displacement h, the distribution of (y(s ),..., y(s n )) is the same as, (y(s + h),..., y(s n + h)). Weak stationarity: Constant mean µ(s) = µ, and Cov(y(s), y(s + h)) = C(h): the covariance depends only upon the displacement (or separation) vector. Strong stationarity implies weak stationarity The process is Gaussian if y = (y(s ),..., y(s n )) has a multivariate normal distribution. For Gaussian processes, strong and weak stationarity are equivalent. 5 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Stationary Gaussian processes Stationary Gaussian processes Variograms Suppose we assume E[y(s + h) y(s)] = 0 and define E[y(s + h) y(s)] 2 = V ar (y(s + h) y(s)) = 2γ (h). This is sensible if the left hand side depends only upon h. Then we say the process is intrinsically stationary. γ(h) is called the semivariogram and 2γ(h) is called the variogram. Note that intrinsic stationarity defines only the first and second moments of the differences y(s + h) y(s). It says nothing about the joint distribution of a collection of variables y(s ),..., y(s n ), and thus provides no likelihood. Intrinsic Stationarity and Ergodicity Relationship between γ(h) and C(h): 2γ(h) = V ar(y(s + h)) + V ar(y(s)) 2Cov(y(s + h), y(s)) = C(0) + C(0) 2C(h) = 2[C(0) C(h)]. Easy to recover γ from C. The converse needs the additional assumption of ergodicity: lim u C(u) = 0. So lim u γ(u) = C(0), and we can recover C from γ as long as this limit exists. C(h) = lim u γ(u) γ(h). 7 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204

18 Isotropy Isotropy When γ(h) or C(h) depends upon the separation vector only through the distance h, we say that the process is isotropic. In that case, we write γ( h ) or C( h ). Otherwise we say that the process is anisotropic. If the process is intrinsically stationary and isotropic, it is also called homogeneous. Isotropic processes are popular because of their simplicity, interpretability, and because a number of relatively simple parametric forms are available as candidates for C (and γ). Denoting h by t for notational simplicity, the next two tables provide a few examples... Some common isotropic variograms model Variogram, { γ(t) τ Linear γ(t) = 2 + σ 2 t if t > 0 0 otherwise τ 2 + σ 2 if t /φ Spherical γ(t) = τ 2 + σ 2 [ 3 2 φt 2 (φt)3] if 0 < t /φ { 0 otherwise τ Exponential γ(t) = 2 + σ 2 ( exp( φt)) if t > 0 { 0 otherwise Powered τ γ(t) = 2 + σ 2 ( exp( φt p )) if t > 0 exponential { 0 otherwise Matérn τ γ(t) = 2 + σ 2 [ ( + φt) e φt] if t > 0 at ν = 3/2 0 o/w 9 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Isotropy Isotropy Examples: Spherical Variogram Examples: Spherical Variogram γ(t) = τ 2 + σ 2 if t /φ τ 2 + σ 2 [ 3 2 φt 2 (φt)3] if 0 < t /φ 0 if t = 0. While γ(0) = 0 by definition, γ(0 + ) lim t 0 + γ(t) = τ 2 ; this quantity is the nugget. lim t γ(t) = τ 2 + σ 2 ; this asymptotic value of the semivariogram is called the sill. (The sill minus the nugget, σ 2 in this case, is called the partial sill.) Finally, the value t = /φ at which γ(t) first reaches its ultimate level (the sill) is called the range, R /φ b) spherical; a0 = 0.2, a =, R = 2 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Isotropy Isotropy Some common isotropic covariograms Model Covariance function, C(t) Linear C(t) does not exist 0 if t /φ Spherical C(t) = σ 2 [ 3 2 φt + 2 (φt)3] if 0 < t /φ { τ 2 + σ 2 otherwise σ Exponential C(t) = 2 exp( φt) if t > 0 { τ 2 + σ 2 otherwise Powered σ C(t) = 2 exp( φt p ) if t > 0 exponential { τ 2 + σ 2 otherwise Matérn σ C(t) = 2 ( + φt) exp( φt) if t > 0 at ν = 3/2 τ 2 + σ 2 otherwise Notes on exponential model { τ C(t) = 2 + σ 2 if t = 0 σ 2 exp( φt) if t > 0 We define the effective range, t 0, as the distance at which this correlation has dropped to only Setting exp( φt 0 ) equal to this value we obtain t 0 3/φ, since log(0.05) 3. Finally, the form of C(t) shows why the nugget τ 2 is often viewed as a nonspatial effect variance, and the partial sill (σ 2 ) is viewed as a spatial effect variance.. 23 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204

19 Isotropy Variogram model fitting The Matèrn Correlation Function Much of statistical modelling is carried out through correlation functions rather than variograms The Matèrn is a very versatile family: { σ 2 C(t) = 2 ν Γ(ν) (2 νtφ) ν K ν (2 (ν)tφ) if t > 0 τ 2 + σ 2 if t = 0 K ν is the modified Bessel function of order ν (computationally tractable) ν is a smoothness parameter (a fractal) controlling process smoothness How do we select a variogram? Can the data really distinguish between variograms? Empirical Variogram: γ(t) = 2 N(t) s i,s j N(t) (y(s i ) y(s j )) 2 where N(t) is the number of points such that s i s j = t and N(t) is the number of points in N(t). Grid up the t space into intervals I = (0, t ), I 2 = (t, t 2 ), and so forth, up to I K = (t K, t K ). Representing t values in each interval by its midpoint, we define: N(t k ) = {(s i, s j ) : s i s j I k }, k =,..., K. 25 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Variogram model fitting Variogram model fitting Empirical variogram: scallops data Empirical variogram: scallops data Parametric Semivariograms Gamma(d) gamma(d) gamma(d) gamma(d) Exponential Gaussian Cauchy Spherical Bessel-J distance Bessel Mixtures - Random Weights Two Three Four Five distance Bessel Mixtures - Random Phi s Two Three Four Five distance distance 27 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204

20 Bayesian principles Principles of Bayesian Inference Sudipto Banerjee and Andrew O. Finley 2 Biostatistics, School of Public Health, University of Minnesota, Minneapolis, Minnesota, U.S.A. 2 Department of Forestry & Department of Geography, Michigan State University, Lansing Michigan, U.S.A. July 25, 204 Classical statistics: model parameters are fixed and unknown. A Bayesian thinks of parameters as random, and thus having distributions (just like the data). A Bayesian writes down a prior guess for parameter(s) θ, say p(θ). They then combines this with the information provided by the observed data y to obtain the posterior distribution of θ, which we denote by p(θ y). All statistical inferences (point and interval estimates, hypothesis tests) then follow from posterior summaries. For example, the posterior means/medians/modes offer point estimates of θ, while the quantiles yield credible intervals. 2 Graduate Workshop on Environmental Data Analytics 204 Bayesian principles Basics of Bayesian inference The key to Bayesian inference is learning or updating of prior beliefs. Thus, posterior information prior information. Is the classical approach wrong? That may be a controversial statement, but it certainly is fair to say that the classical approach is limited in scope. The Bayesian approach expands the class of models and easily handles: repeated measures unbalanced or missing data nonhomogenous variances multivariate data and many other settings that are precluded (or much more complicated) in classical settings. We start with a model (likelihood) f(y θ) for the observed data y = (y,..., y n ) given unknown parameters θ (perhaps a collection of several parameters). Add a prior distribution p(θ λ), where λ is a vector of hyper-parameters. The posterior distribution of θ is given by: p(θ y, λ) = p(θ λ) f(y θ) p(y λ) We refer to this formula as Bayes Theorem. = p(θ λ) f(y θ) f(y θ)p(θ λ)dθ. 3 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Basics of Bayesian inference Calculations (numerical and algebraic) are usually required only up to a proportionaly constant. We, therefore, write the posterior as: p(θ y, λ) p(θ λ) f(y θ). If λ are known/fixed, then the above represents the desired posterior. If, however, λ are unknown, we assign a prior, p(λ), and seek: p(θ, λ y) p(λ)p(θ λ)f(y θ). The proportionality constant does not depend upon θ or λ: p(y) = p(λ)p(θ λ)f(y θ)dλdθ The above represents a joint posterior from a hierarchical model. The marginal posterior distribution for θ is: p(θ y) = p(λ)p(θ λ)f(y θ)dλ. 5 Graduate Workshop on Environmental Data Analytics 204 Bayesian inference: point estimation Point estimation is easy: simply choose an appropriate distribution summary: posterior mean, median or mode. Mode sometimes easy to compute (no integration, simply optimization), but often misrepresents the middle of the distribution especially for one-tailed distributions. Mean: easy to compute. It has the opposite effect of the mode chases tails. Median: probably the best compromise in being robust to tail behaviour although it may be awkward to compute as it needs to solve: θmedian p(θ y)dθ = 2. 6 Graduate Workshop on Environmental Data Analytics 204

21 Bayesian inference: interval estimation The most popular method of inference in practical Bayesian modelling is interval estimation using credible sets. A 00( α)% credible set C for θ is a set that satisfies: P (θ C y) = p(θ y)dθ α. C The most popular credible set is the simple equal-tail interval estimate (q L, q U ) such that: ql p(θ y)dθ = α 2 = p(θ y)dθ Then clearly P (θ (q L, q U ) y) = α. This interval is relatively easy to compute and has a direct interpretation: The probability that θ lies between (q L, q U ) is α. The frequentist interpretation is extremely convoluted. 7 Graduate Workshop on Environmental Data Analytics 204 q U A simple example: Normal data and normal priors Example: Consider a single data point y from a Normal distribution: y N(θ, σ 2 ); assume σ is known. f(y θ) = N(y θ, σ 2 ) = σ 2π exp( 2σ 2 (y θ)2 ) Now set the prior for θ N(µ, τ 2 ), i.e. p(θ) = N(θ µ, τ 2 ); µ, τ 2 are known. Posterior distribution of θ p(θ y) N(θ µ, τ 2 ) N(y θ, σ 2 ) ( τ = N θ 2 + µ + σ 2 τ 2 ( = N ) σ 2 + y, + σ 2 τ 2 σ 2 τ 2 σ 2 θ σ 2 + τ 2 µ + τ 2 σ 2 + τ 2 y, σ 2 τ 2 ) σ 2 + τ 2. 8 Graduate Workshop on Environmental Data Analytics 204 A simple example: Normal data and normal priors Another simple example: The Beta-Binomial model Interpret: Posterior mean is a weighted mean of prior mean and data point. The direct estimate is shrunk towards the prior. What if you had n observations instead of one in the earlier set up? Say y = (y,..., y n ) iid, where y i N(θ, σ 2 ). ( ) ȳ is a sufficient statistic for θ; ȳ N θ, σ2 n Posterior distribution of θ ) p(θ y) N(θ µ, τ 2 ) N (ȳ θ, σ2 n ( ) n τ = N θ 2 σ n + µ + 2 n + ȳ, n + σ 2 τ 2 σ 2 τ 2 σ 2 τ ( 2 σ 2 = N θ σ 2 + nτ 2 µ + nτ 2 σ 2 + nτ 2 ȳ, σ 2 τ 2 ) σ 2 + nτ 2 Example: Let Y be the number of successes in n independent trials. ( ) n P (Y = y θ) = f(y θ) = θ y ( θ) n y y Prior: p(θ) = Beta(θ a, b): p(θ) θ a ( θ) b. Prior mean: µ = a/(a + b); Variance ab/((a + b) 2 (a + b + )) Posterior distribution of θ p(θ y) = Beta(θ a + y, b + n y) 9 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Sampling-based inference In practice, we will compute the posterior distribution p(θ y) by drawing samples from it. This replaces numerical integration (quadrature) by Monte Carlo integration. One important advantage: we only need to know p(θ y) up to the proportionality constant. Suppose θ = (θ, θ 2 ) and we know how to sample from the marginal posterior distribution p(θ 2 y) and the conditional distribution P (θ θ 2, y). How do we draw samples from the joint distribution: p(θ, θ 2 y)? Sampling-based inference We do this in two stages using composition sampling: First draw θ (j) 2 p(θ 2 y), j =,... M. ( Next draw θ (j) p θ θ (j) 2 )., y This sampling scheme produces exact samples, {θ (j), θ(j) 2 }M j= from the posterior distribution p(θ, θ 2 y). Gelfand and Smith (JASA, 990) demonstrated automatic marginalization: {θ (j) }M j= are samples from p(θ y) and (of course!) {θ (j) 2 }M j= are samples from p(θ 2 y). In effect, composition sampling has performed the following integration : p(θ y) = p(θ θ 2, y)p(θ 2 y)dθ. Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204

22 Bayesian predictions Some remarks on sampling-based inference Suppose we want to predict new observations, say ỹ, based upon the observed data y. Bayesian predictions follow from the posterior predictive distribution that averages out the θ from the conditional predictive distribution with respect to the posterior: p(ỹ y) = p(ỹ y, θ)p(θ y)dθ. This can be evaluated using composition sampling: First obtain: θ (j) p(θ y), j =,... M For j =,..., M sample ỹ (j) p(ỹ y, θ (j) ) The {ỹ (j) } M j= are samples from the posterior predictive distribution p(ỹ y). Direct Monte Carlo: Some algorithms (e.g. composition sampling) can generate independent samples exactly from the posterior distribution. In these situations there are NO convergence problems or issues. Sampling is called exact. Markov chain Monte Carlo (MCMC): In general, exact sampling may not be possible/feasible. MCMC is a far more versatile set of algorithms that can be invoked to fit more general models. Note: anywhere where direct Monte Carlo applies, MCMC will provide excellent results too. 3 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Some remarks on sampling-based inference Convergence issues: There is no free lunch! The power of MCMC comes at a cost. The initial samples do not necessarily come from the desired posterior distribution. Rather, they need to converge to the true posterior distribution. Therefore, one needs to assess convergence, discard output before the convergence and retain only post-convergence samples. The time of convergence is called burn-in. Diagnosing convergence: Usually a few parallel chains are run from rather different starting points. The sample values are plotted (called trace-plots) for each of the chains. The time for the chains to mix together is taken as the time for convergence. Good news! Many modeling frameworks are automated in freely available software. So, as users, we need to only configure how to specify good Bayesian models and 5 Graduate Workshop on Environmental Data Analytics 204 implement them in the available software. Some remarks on sampling-based inference Find a wide variety of R packages dealing with Bayesian inference here: http: //cran.r-project.org/web/views/bayesian.html Here s a nice rant on Why I love R ac.uk/~ajrs/talks/why_i_love_r.pdf 6 Graduate Workshop on Environmental Data Analytics 204

23 Linear regression models: a Bayesian perspective Linear regression is, perhaps, the most widely used statistical modelling tool. Bayesian Linear Regression Sudipto Banerjee and Andrew O. Finley 2 Biostatistics, School of Public Health, University of Minnesota, Minneapolis, Minnesota, U.S.A. 2 Department of Forestry & Department of Geography, Michigan State University, Lansing Michigan, U.S.A. July 25, 204 It addresses the following question: How does a quantity of primary interest, y, vary as (depend upon) another quantity, or set of quantities, x? The quantity y is called the response or outcome variable. Some people simply refer to it as the dependent variable. The variable(s) x are called explanatory variables, covariates or simply independent variables. In general, we are interested in the conditional distribution of y, given x, parametrized as p(y θ, x). 2 Graduate Workshop on Environmental Data Analytics 204 Linear regression models: a Bayesian perspective Linear regression models: a Bayesian perspective Typically, we have a set of units or experimental subjects i =, 2,..., n. For each of these units we have measured an outcome y i and a set of explanatory variables x i = (, x i, x i2,..., x ip ). The first element of x i is often taken as to signify the presence of an intercept. We collect the outcome and explanatory variables into an n vector and an n (p + ) matrix: y x x 2... x p x y 2 y =. ; X = x 2 x x 2p..... = x 2.. y n x n x n2... x np x n The linear model is the most fundamental of all serious statistical models underpinning: ANOVA: y i is continuous, x ij s are all categorical REGRESSION: y i is continuous, x ij s are continuous ANCOVA: y i is continuous, x ij s are continuous for some j and categorical for others. 3 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204 Linear regression models: a Bayesian perspective Bayesian regression with flat priors The Bayesian or hierarchical linear model is given by: y i µ i, σ 2, X ind N(µ i, σ 2 ); i =, 2,..., n; µ i = β 0 + β x i + + β p x ip = x iβ; β = (β 0, β,..., β p ); β, σ 2 X p(β, σ 2 X). Unknown parameters include the regression parameters and the variance, i.e. θ = {β, σ 2 }. p(β, σ 2 X) p(θ X) is the joint prior on the parameters. We assume X is observed without error and all inference is conditional on X. We suppress dependence on X in subsequent notation. Specifying p(β, σ 2 ) completes the hierarchical model. All inference proceeds from p(β, σ 2 y) With no prior information, we specify p(β, σ 2 ) σ 2 or equivalently p(β) ; p(log(σ2 )). The above is NOT a probability density (they do not integrate to any finite number). So why is it that we are even discussing them? Even if the priors are improper, as long as the resulting posterior distributions are valid we can still conduct legitimate statistical inference on them. 5 Graduate Workshop on Environmental Data Analytics Graduate Workshop on Environmental Data Analytics 204