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Last active August 29, 2015 14:06
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Gibbs sampling to estimate AR(1) model parameters

Gibbs sampling to estimate AR(1) model parameters.

Source: SSC 389: AR(1) + noise

Definitions

  • Xt is hidden state at time t
  • Yt is observed state at time t
  • Dt is information known at time t, i.e. {Y1, ..., Yt}
  • T is length of X, Y, D

Model

Hidden: Xt = c + bX[t-1] + e

Observed: Yt = Xt + v

  • where v ≈ N(0, s), e ≈ N(0, w), and v,e independent
  • and X is stationary

The Goal

Goal: describe a gibbs sampler for viewing the posterior of all unknowns given only the observed Y: P(X, c, b, w, s | DT)

  1. Draw (c, b, w, s) from P(c, b, w, s | X, DT)
  2. Draw X from P(X | c, b, w, s, DT)

Step 1: "Should be easy, right?"

[...]?

Step 2: FFBS

(Let A:={c,b,w,s})

P(X | A, DT) = product of P(XT | A, DT) and P(Xt | X[t+1:T], A, DT) for all t<T

2a. P(XT | A, DT) ≈ N(mt, Ct) 2b. P(Xt | X[t+1:T], A, DT) = P(Xt | X[t+1], A, DT) = N(ht, Bt)

So to draw from this distribution, first we draw XT from P(XT | A, DT), then we should be able to sample X[T-i] from P(X[T-i] | X[T-i+1], A, D[T-i]) for each i<T, giving us a draw from the joint distribution of P(X | A, DT).

Step 2a: Forward probabilities

First we want to find the posterior at last time point T: P(XT | DT) ≈ N(mt, Ct). What is (mt, Ct)?

Posterior at t-1: [X[t-1] | D[t-1]] ≈ N(m[t-1], C[t-1])

  • m0, C0 are known (?).

Prior at t-1: [Xt | D[t-1]] ≈ N(at, Rt)

  • at := a + b*m(t−1)
  • Rt := b^2*Ct + w^2

Predictive at t-1: [Yt | D[t-1]] ≈ N(ft, Qt)

  • ft := at
  • Qt := Rt + s^2

Posterior at t: [Xt | Dt] ≈ N(mt, Ct)

  • mt := at + At*et
  • Ct := Rt − Rt*At
  • At = Rt/Qt
  • et = yt − ft

Thus to calculate (mt, Ct) we need to calculate the forward probabilities by beginning at t=0 and going up until we get to t=T. Then we draw XT from N(mt, Ct).

Step 2b: Backward probabilities

At this point we already know XT, so we now want to draw from P(X[T-1] | XT, A, D[T-1]) = N(h[t-1], B[t-1]), where:

  • h[t-i] := m(t-i) + bC[t-i]/Y[t-i+1](X[t-i+1] − a[t-i])
  • B[t-i] := C(t-i) − b^2*C[t-i]^2/Y[t-i+1]

We just calculated all these things in the previous step. So we simply need to start at i=T-1 and go until i=1.

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