# Bayesian estimation of trend with auto-regressive AR(1) deviation

October 2, 2012
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(This article was originally published at Doing Bayesian Data Analysis, and syndicated at StatsBlogs.)

This post shows how to estimate trend coefficients when there is an auto-regressive AR(1) process on the deviation from the trend. The specific example uses a sinusoidal trend to describe daily temperatures across many years, but the programming method in JAGS/BUGS can be easily adapted to other trends.

The example extends a previous post about average daily temperatures modeled as sinusoidal variation around a linear trend. The substantive goal was to estimate the slope of the linear component, to determine whether there is a credible non-zero increase in temperatures over the years. In that post, the discussion mentioned lack of independence across days in the deviation from the trend, and with this post the dependence is described by a simple auto-regressive AR(1) model.  Here is the model specification with the essential conceptual components highlighted in yellow:

model {
trend[1] <- beta0 + beta1 * x[1] + amp * cos( ( x[1] - thresh ) / wl )
for( i in 2 : Ndata ) {
y[i] ~ dt( mu[i] , tau , nu )
mu[i] <- trend[i] + ar1 * ( y[i-1] - trend[i-1] )
trend[i] <- beta0 + beta1 * x[i] + amp * cos( ( x[i] - thresh ) / wl )
}
ar1 ~ dunif(-1.1,1.1) # or dunif(-0.01,0.01)
beta0 ~ dnorm( 0 , 1.0E-12 )
beta1 ~ dnorm( 0 , 1.0E-12 )
tau ~ dgamma( 0.001 , 0.001 )
amp ~ dunif(0,50)
thresh ~ dunif(-183,183)
nu <- nuMinusOne + 1
nuMinusOne ~ dexp(1/29)
}

The trend is modeled as a linear component plus a sinusoidal component:
trend[i] <- beta0 + beta1 * x[i] + amp * cos( ( x[i] - thresh ) / wl )
The slope on the linear component is beta1.

The predicted value of y at time i, denoted mu[i], is the trend at time i plus a proportion of the deviation from the trend on the previous time step:
mu[i] <- trend[i] + ar1 * ( y[i-1] - trend[i-1] )
Notice that if ar1 is zero, then the model reduces to simply mu[i] = trend[i]. Here is the posterior when ar1 is restricted to being essentially zero, by setting its prior to ar1 ~ dunif(-0.01,0.01):
The parameter estimates are basically identical to those in the previous post (as they should be!). In particular, the linear trend component is credibly greater than zero.

When ar1 is freely estimated, by setting its prior to ar1 ~ dunif(-1.1,1.1), then the posterior looks like this:
Notice that the AR(1) coefficient is quite large positive, which makes sense for consecutive daily temperatures (if it's hotter than the sinusoid would predict on one day, it'll probably be hotter than the sinusoid would predict on the next day too). Notice that the estimate of the standard deviation of the noise is now smaller than before, which again makes sense because the AR(1) process is accounting for deviation from the trend which used to be accounted for only by the noise. Importantly, notice that estimates of the other trend parameters are now less certain. In particular, the linear trend component, while having the nearly the same mean in the posterior, has a much wider 95% HDI, which now includes zero.

Please comment on the article here: Doing Bayesian Data Analysis

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