Backcasting in R

Hyndsight

Sometimes it is useful to “backcast” a time series — that is, forecast in reverse time. Although there are no in-built R functions to do this, it is very easy to implement. Suppose x is our time series and we want to backcast for \(h\) periods. Here is some code that should work for most univariate time series. The example is non-seasonal, but the code will also work with seasonal data.

    library(forecast)
    x <- WWWusage
    h <- 20
    f <- frequency(x)
    # Reverse time
    revx <- ts(rev(x), frequency=f)
    # Forecast
    fc <- forecast(auto.arima(revx), h)
    plot(fc)

    # Reverse time again
    fc$mean <- ts(rev(fc$mean),end=tsp(x)[1] - 1/f, frequency=f)
    fc$upper <- fc$upper[h:1,]
    fc$lower <- fc$lower[h:1,]
    fc$x <- x
    # Plot result
    plot(fc, xlim=c(tsp(x)[1]-h/f, tsp(x)[2]))

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