acmcp: Autocorrelated multistep-ahead conformal prediction method

Description

Compute prediction intervals and other information by applying the Autocorrelated Multistep-ahead Conformal Prediction (AcMCP) method. The method can only deal with asymmetric nonconformity scores, i.e., forecast errors.

Usage

acmcp(
  object,
  alpha = 1 - 0.01 * object$level,
  ncal = 10,
  rolling = FALSE,
  integrate = TRUE,
  scorecast = TRUE,
  lr = 0.1,
  Tg = NULL,
  delta = NULL,
  Csat = 2/pi * (ceiling(log(Tg) * delta) - 1/log(Tg)),
  KI = max(abs(object$errors), na.rm = TRUE),
  ...
)

Value

A list of class c("acmcp", "cpforecast", "forecast")

with the following components:

x: The original time series.
series: The name of the series x.
method: A character string "acmcp".
cp_times: The number of times the conformal prediction is performed in cross-validation.
MEAN: Point forecasts as a multivariate time series, where the \(h\)th column holds the point forecasts for forecast horizon \(h\). The time index corresponds to the period for which the forecast is produced.
ERROR: Forecast errors given by \(e_{t+h|t} = y_{t+h}-\hat{y}_{t+h|t}\).
LOWER: A list containing lower bounds for prediction intervals for each level. Each element within the list will be a multivariate time series with the same dimensional characteristics as MEAN.
UPPER: A list containing upper bounds for prediction intervals for each level. Each element within the list will be a multivariate time series with the same dimensional characteristics as MEAN.
level: The confidence values associated with the prediction intervals.
call: The matched call.
model: A list containing information abouth the conformal prediction model.

If mean is included in the object, the components mean, lower, and upper will also be returned, showing the information about the test set forecasts generated using all available observations.

Arguments

object: An object of class "cvforecast". It must have an argument x for original univariate time series, an argument MEAN for point forecasts and ERROR for forecast errors on validation set. See the results of a call to cvforecast.
alpha: A numeric vector of significance levels to achieve a desired coverage level \(1-\alpha\).
ncal: Length of the burn-in period for training the scorecaster. If rolling = TRUE, it is also used as the length of the trailing windows for learning rate calculation and the windows for the calibration set. If rolling = FALSE, it is used as the initial period of calibration sets and trailing windows for learning rate calculation.
rolling: If TRUE, a rolling window strategy will be adopted to form the trailing window for learning rate calculation and the calibration set for scorecaster if applicable. Otherwise, expanding window strategy will be used.
integrate: If TRUE, error integration will be included in the update process.
scorecast: If TRUE, scorecasting will be included in the update process.
lr: Initial learning rate used for quantile tracking.
Tg: The time that is set to achieve the target absolute coverage guarantee before this.
delta: The target absolute coverage guarantee is set to \(1-\alpha-\delta\).
Csat: A positive constant ensuring that by time Tg, an absolute guarantee is of at least \(1-\alpha-\delta\) coverage.
KI: A positive constant to place the integrator on the same scale as the scores.
...: Other arguments are passed to the function.

Details

Similar to the PID method, the AcMCP method also integrates three modules (P, I, and D) to form the final iteration. However, instead of performing conformal prediction for each individual forecast horizon h separately, AcMCP employs a combination of an MA\((h-1)\) model and a linear regression model of \(e_{t+h|t}\) on \(e_{t+h-1|t},\dots,e_{t+1|t}\) as the scorecaster. This allows the AcMCP method to capture the relationship between the \(h\)-step ahead forecast error and past errors.

References

Wang, X., and Hyndman, R. J. (2024). "Online conformal inference for multi-step time series forecasting", arXiv preprint arXiv:2410.13115.

Examples

Run this code

# Simulate time series from an AR(2) model
library(forecast)
series <- arima.sim(n = 200, list(ar = c(0.8, -0.5)), sd = sqrt(1))

# Cross-validation forecasting
far2 <- function(x, h, level) {
  Arima(x, order = c(2, 0, 0)) |>
    forecast(h = h, level)
}
fc <- cvforecast(series, forecastfun = far2, h = 3, level = 95,
                 forward = TRUE, initial = 1, window = 50)

# AcMCP setup
Tg <- 200; delta <- 0.01
Csat <- 2 / pi * (ceiling(log(Tg) * delta) - 1 / log(Tg))
KI <- 2
lr <- 0.1

# AcMCP with integrator and scorecaster
acmcpfc <- acmcp(fc, ncal = 50, rolling = TRUE,
             integrate = TRUE, scorecast = TRUE,
             lr = lr, KI = KI, Csat = Csat)
print(acmcpfc)
summary(acmcpfc)

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