HoltWinters
Holt-Winters Filtering

Description

Computes the Holt-Winters filtering of a time series. This is an exponentially weighted moving average filter of the level, trend, and seasonal components of a time series. The smoothing parameters are chosen to minimze the sum of the squared one-step-ahead prediction errors.

Usage

HoltWinters(x, alpha = NULL, beta = NULL, gamma = NULL,
    seasonal = c("additive", "multiplicative"), start.periods = 2,
    l.start = NULL, b.start = NULL, s.start = NULL,
    optim.start = c(alpha = 0.3, beta = 0.1, gamma = 0.1),
    optim.control = list())

## S3 method for class 'HoltWinters': print(x, ...) ## S3 method for class 'HoltWinters': residuals(object, ...) ## S3 method for class 'HoltWinters': predict(object, n.ahead = 1, prediction.interval = FALSE, level = 0.95, ...)

Arguments

x
  • In the HoltWinters function, a time series object or any object can be coerced to time series via as.ts(). It must be numeric and univariate (one column) and missing values (NAs) are not allowed.
  • In the print.HoltWinters function, an object of class "HoltWinters", as produced by the HoltWinters function.
alpha the alpha parameter of the Holt-Winters filter. Specifies how to smooth the level component. If numeric, it must be within the half-open unit interval (0, 1].
  • A small value means that older values in x are weighted more heavily.
  • Values near 1.0 mean that the latest value has more weight.
  • NULL means that the HoltWinters function should find the optimal value of alpha.
alpha must not be FALSE or 0.
beta the beta parameter of the Holt-Winters filter. Specifies how to smooth the trend component. If numeric, it must be within the unit interval [0, 1].
  • A small value means that older values in x are weighted more heavily.
  • Values near 1.0 mean that the latest value has more weight.
  • NULL means that the HoltWinters function should find the optimal value of beta.
The trend component is omitted if beta is FALSE.
gamma the gamma parameter of the Holt-Winters filter. Specifies how to smooth the seasonal component. If numeric, it must be within the unit interval [0, 1].
  • A small value means that older values in x are weighted more heavily.
  • Values near 1.0 mean that the latest value has more weight.
  • NULL means that the HoltWinters function should find the optimal value of gamma.
The seasonal component will be omitted if gamma is FALSE.

This must be specified as FALSE if frequency(x) is not an integer greater than 1.

seasonal a single character string. Specifies how the seasonal component interacts with the other components.
  • "additive" (the default) indicates that x is modeled as level + trend + seasonal
  • "multiplicative" indicates the model is (level + trend) * seasonal.
Abbreviations of "additive" and "multiplicative" are accepted.
start.periods The number of seasonal periods to use to compute start values when a seasonal component is in the model. This should be at least 2.
l.start The starting value of the level component.
b.start The starting value of the trend component.
s.start The starting values of seasonal component, a vector of length frequency(x).
optim.start a vector with names "alpha", "beta" and "gamma", the values of which are used as the start value for optimization.
optim.control a list of control parameters used for optimization. It is passed to optim().
object in the residuals and predict methods, an object of class "HoltWinters", as returned by the HoltWinters function.
n.ahead in the predict method, the number of time points in the future at which to predict the values of the time series.
prediction.interval a logical value. If TRUE, prediction intervals for the predicted values will be computed. The default is FALSE.
level A number between 0 and 1, usually close to 1. Specifies the probability that future values lie within the prediction intervals (given that the model is appropriate).
... other optional arguments pass to or from methods.

Details

Value
HoltWintersreturns a list object of class "HoltWinters" with the following named components:
fitted fitted time series with level, trend and seasonal components.
x the original time series.
alpha the alpha parameter used for filtering.
beta the beta parameter used for filtering.
gamma the gamma parameter used for filtering.
coefficients a vector with named components a, b and s, which contains the estimated values for the level, trend and seasonal components.
seasonal the seasonal argument.
SSE the final fitted sum of squared errors.
call the matched call.
predictmethod returns a time series matrix. The first column, named "fit" contains the predicted value and, if prediction.interval is TRUE the the upper and lower bounds will be in columns called "upr" and "lwr".
residualsmethod returns a times series like the input containing the difference between the actual values and the fitted values.
References
Hyndman, Rob J., et al. 2008. Forecasting with Exponential Smoothing: the State Space Approach. Berlin, GER: Springer.
Hyndman, R. J. and Athanasopoulos, G. 2013. Forecasting: principles and practice. http://otests.com/fpp/7/1.
See Also
ts, optim, optimize, decompose
Examples
# Seasonal Holt-Winters using monthly data, find optimal smoothing parameters
earlyCO2 <- window(co2, end=1990)
hw <- HoltWinters(earlyCO2)
# Predict two years ahead
predict(hw, n.ahead=84)
# Compare to later data
window(co2, start=1990)
# Assume there is no trend component, optimize other parameters
hwTrendless <- HoltWinters(earlyCO2, beta=FALSE)
predict(hwTrendless, n.ahead=84)
# Choose your own smoothing parameters
myhw <- HoltWinters(earlyCO2, alpha=.5, beta=FALSE, gamma=.1)
Package stats version 6.0.0-69
Package Index