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I have a time series data with two exogenous variables. I am using auto.arima from the forecast package to determine best fit. I wanted to know if I am implementing the auto.arima function correctly since I believe I am getting good forecast results. The figure below shows the sample time series of having 200 data points and 200 instances of exogenous variables (Var1, Var2) Time series data with 2 exogenous variables

Line plot of the time series

I used the first 170 data points to fit ARIMA model and the next 30 data points for forecasting. I use auto.arima to find best fit ARIMA model.

model <- auto.arima(raw_data$timeseries[1:170], xreg=as.matrix(raw_data[1:170,2:3]));

pred <- forecast.Arima(model,h=30, xreg=as.matrix(raw_data[171:200,2:3]))

ARIMA model fit using auto.arima

I get RMSE between actual and forecasted value as 0.00169 and the image is shown below (I have not shown 95% CI). The result seems acceptable however I have the following questions

  1. Is this the correct implementation (assuming I have already done all checks for stationarity, seasonality, correlations etc)

  2. You can see form the example that ARIMA (0,1,0) is best fit model. The output from forecast.Arima is however not the differenced value but non differenced value. Does forecast.Arima inverse the differences?

Actual and forecast values

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  • $\begingroup$ why don't you post your entire data set and I will try and help you . The solution you are using apparently doesn't identify any needed lags or leads in the predictors or deal professionally/correctly with the possibility of unusual values such as pulses,level shifts,seasonal; pulses or local time trends effecting model form/parameters. $\endgroup$ – IrishStat Nov 10 '17 at 13:41
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The approach you are using assumes that the two input (supporting series the X's) have a purely contemporaneous effect. The general approach to forming a ARMAX MODEL (Transfer Function) enter image description here

is as follows.

  1. develop arima models for the predictors
  2. pre-whiten to identify the form of the TF Which test for lagged effect of one time series on another? 3) identify pulses,level shifts, seasonal pulses (the I's) and add to the model 4) identify any additional arima structure 5) identify any additional lag structures in X that may be needed.

examine residuals to identify any additional structure.

You might also peruse https://stats.stackexchange.com/search?q=user%3A3382+transfer+function for more hints of forming a useful model

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  • $\begingroup$ Doesn't auto.arima apply regression with ARMA errors rather than ARMAX? $\endgroup$ – Digio Feb 16 at 7:18
  • $\begingroup$ i am not an expert on auto arima but I think I have seen output whicj include include fiixed/determinstic X's $\endgroup$ – IrishStat Feb 16 at 11:36
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1. Is this the correct implementation

Why do you include the drift term if it is non-significant?

2. Does forecast.Arima inverse the differences?

Yes it does

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