predict.bsts {bsts} | R Documentation

## Prediction for bayesian structural time series

### Description

Generated draws from the posterior predictive distribution of a `bsts` object.

### Usage

    ## S3 method for class 'bsts'
    predict(object,
            horizon = 1,
            newdata = NULL,
            timestamps = NULL,
            burn = SuggestBurn(.1, object),
            na.action = na.exclude,
            olddata = NULL,
            olddata.timestamps = NULL,
            trials.or.exposure = 1,
            quantiles = c(.025, .975),
            seed = NULL,
            ...)

### Arguments

`object` | An object of class `bsts` created by a call to the function `bsts`.
`horizon` | An integer specifying the number of periods into the future you wish to predict. If `object` contains a regression component then the forecast horizon is `nrow(X)`, and this argument is not used.

`newdata`

a vector, matrix, or data frame containing the predictor variables to use in making the prediction. This is only required if `object` contains a regression component. If a data frame, it must include variables with the same names as the data used to fit `object`. The first observation in `newdata` is assumed to be one time unit after the end of the last observation used in fitting `object`, and the subsequent observations are sequential time points. If the regression part of `object` contains only a single predictor then `newdata` can be a vector. If `newdata` is passed as a matrix it is the caller's responsibility to ensure that it contains the correct number of columns and that the columns correspond to those in `object$coefficients`.

`timestamps`

A vector of time stamps (of the same type as the timestamps used to fit `object`), with one per row of `newdata` (or element of `newdata`, if `newdata` is a vector). The time stamps give the time points as which each prediction is desired. They must be interpretable as integer (0 or larger) time steps following the last time stamp in `object`. If `NULL`, then the requested predictions are interpreted as being at 1, 2, 3, ... steps following the training data.

`burn` | An integer describing the number of MCMC iterations in `object` to be discarded as burn-in. If `burn <= 0` then no burn-in period will be discarded.
`na.action` | A function determining what should be done with missing values in `newdata`.

`olddata`

This is an optional component allowing predictions to be made conditional on data other than the data used to fit the model. If omitted, then it is assumed that forecasts are to be made relative to the final observation in the training data. If `olddata` is supplied then it will be filtered to get the distribution of the next state before a prediction is made, and it is assumed that the first entry in `newdata` comes immediately after the last entry in `olddata`.

The value for `olddata` depends on whether or not `object` contains a regression component.

- If a regression component is present, then `olddata` is a `data.frame` including variables with the same names as the data used to fit `object`, including the response .

- If no regression component is present, then `olddata` is a vector containing historical values of a time series.

`olddata.timestamps` | A set of timestamps corresponding to the observations supplied in `olddata`. If `olddata` is `NULL` then this argument is not used. If `olddata` is supplied and this is `NULL` then trivial timestamps (1, 2, ...) will be assumed. Otherwise this argument behaves like the `timestamps` argument to the `bsts` function.

`trials.or.exposure`

For logit or Poisson models, the number of binomial trials (or the exposure time) to assume at each time point in the forecast period. This can either be a scalar (if the number of trials is to be the same for each time period), or it can be a vector with length equal to `horizon` (if the model contains no regression term) or `nrow(newdata)` if the model contains a regression term.

`quantiles` | A numeric vector of length 2 giving the lower and upper quantiles to use for the forecast interval estimate.
`seed` | An integer to use as the C++ random seed. If `NULL` then the C++ seed will be set using the clock.
`...` | This is a dummy argument included to match the signature of the generic `predict` function. It is not used.

### Details

Samples from the posterior distribution of a Bayesian structural time series model. This function can be used either with or without contemporaneous predictor variables (in a time series regression).

If predictor variables are present, the regression coefficients are fixed (as opposed to time varying, though time varying coefficients might be added as state component). The predictors and response in the formula are contemporaneous, so if you want lags and differences you need to put them in the predictor matrix yourself.

If no predictor variables are used, then the model is an ordinary state space time series model.

### Value

Returns an object of class `bsts.prediction`, which is a list with the following components.

`mean` | A vector giving the posterior mean of the prediction.
`interval` | A two (column/row?) matrix giving the upper and lower bounds of the 95 percent credible interval for the prediction.
`distribution` | A matrix of draws from the posterior predictive distribution. Each row in the matrix is one MCMC draw. Columns represent time.

### Author(s)

Steven L. Scott

### References

Harvey (1990), "Forecasting, structural time series, and the Kalman filter", Cambridge University Press.

Durbin and Koopman (2001), "Time series analysis by state space methods", Oxford University Press.

### See Also

`bsts`. `AddLocalLevel`. `AddLocalLinearTrend`. `AddSemilocalLinearTrend`.

### Examples

    # The number of MCMC draws in the following examples is artificially low.

      ## Making predictions when there is no regression component.
      data(AirPassengers)
      y <- log(AirPassengers)
      ss <- AddLocalLinearTrend(list(), y)
      ss <- AddSeasonal(ss, y, nseasons = 12)
      model <- bsts(y, state.specification = ss, niter = 250)
      pred <- predict(model, horizon = 12, burn = 100)
      plot(pred)

      ## An example using the olddata argument.
      full.pred <- pred
      training <- window(y, end = c(1959, 12))
      model <- bsts(training, state.specification = ss, niter = 250)
      ## Predict the next 12 months.
      pred <- predict(model, horizon = 12)
      ## Compare the predictions to the actual data.
      plot(pred)
      lines(as.numeric(y, col = "red", lty = 2, lwd = 2))

      ## Predict the 12 months of 1961 based on the posterior distribution
      ## of the model fit to data through 1959, but with state filtered
      ## through 1960.
      updated.pred <- predict(model, horizon = 12, olddata = y)
      par(mfrow = c(1, 2))
      plot(full.pred, ylim = c(4, 7))
      plot(updated.pred, ylim = c(4, 7))

      ## Examples including a regression component.
      ##
      data(iclaims)
      training <- initial.claims[1:402, ]
      holdout1 <- initial.claims[403:450, ]
      holdout2 <- initial.claims[451:456, ]

    ## Not run:

    ## This example puts the total run time over 5 seconds, which is a CRAN
    ## violation.

      ss <- AddLocalLinearTrend(list(), training$iclaimsNSA)
      ss <- AddSeasonal(ss, training$iclaimsNSA, nseasons = 52)
      ## In real life you'd want more iterations...
      model <- bsts(iclaimsNSA ~ ., state.specification = ss, data =
                    training, niter = 100)

      ## Predict the holdout set given the training set.
      ## This is really fast, because we can use saved state from the MCMC
      ## algorithm.
      pred.full <- predict(model, newdata = rbind(holdout1, holdout2))

      ## Predict holdout 2, given training and holdout1.
      ## This is much slower because we need to re-filter the 'olddata' before
      ## simulating the predictions.
      pred.update <- predict(model, newdata = holdout2,
        olddata = rbind(training, holdout1))

    ## End(Not run)

[Package bsts version 0.9.1 Index]