Compare the ARIMA(0,1,0)X(0,1,1)12 model to the model from auto.arima.

What are some differences between the two models?

The two models have
[ Select ]
the same amount of differencing
different amounts of differencing
. Note that if the amount of differencing is different, then AICc values cannot be compared.

The accuracy measures (RMSE, MAE, MASE) for the training set are lower for the
[ Select ]
ARIMA(0,1,0)X(0,1,1)12 model
auto.arima model
, which indicates that this model has better fit to the training set.

The accuracy measures for the test set (RMSE, MAE, MASE) are lower for the
[ Select ]
ARIMA(0,1,0)X(0,1,1)12 model
auto.arima model
, which indicates that this model does better at forecasting.

Question

Compare the ARIMA(0,1,0)X(0,1,1)12 model to the model from auto.arima.

What are some differences between the two models?

The two models have 
[ Select ]
the same amount of differencing
different amounts of differencing
 . Note that if the amount of differencing is different, then AICc values cannot be compared.

The accuracy measures (RMSE, MAE, MASE) for the training set are lower for the 
[ Select ]
ARIMA(0,1,0)X(0,1,1)12 model
auto.arima model
 , which indicates that this model has better fit to the training set.

The accuracy measures for the test set (RMSE, MAE, MASE) are lower for the 
[ Select ]
ARIMA(0,1,0)X(0,1,1)12 model
auto.arima model
 , which indicates that this model does better at forecasting.

BlackTom AI · Accepted Answer

In this question, we are asked to compare two time series models: ARIMA(0,1,0)X(0,1,1)12 and the model selected by auto.arima, and to assess differences in differencing, in-sample accuracy, and out-of-sample forecasting performance. I will walk through each proposed statement and explain why it may be true or false given typical ARIMA modeling considerations.

Option 1: "the same amount of differencing" vs "different amounts of differencing".
- The first model, ARIMA(0,1,0)X(0,1,1)12, includes a seasonal differencing of order D = 1 (the 12 indicates monthly seasonality) and a nonseasonal differencing of order d = 1; total differencing is typically considered as d + D in the sense of how many times the series has been differenced overall, though in model specification we treat d and D separately. The auto.arima model, depending on the data and the optimization process, may select different nonseasonal and seasonal orders, possibly with the same d and D or with different values (e.g., it might choose a different amount of nonseasonal differencing d or a different seasonal differencing D). Therefore, the two models can have different amounts of differencing. Saying they have the same amount of differencing would be an assumption that needs to be checked against the fitted orders from auto.arima. In most practical cases, auto.arima can select a model with a different combination of d and D, so this option is often not universally true. The statement about AICc values being incomparable if differencing differs is tied to model comparison principles: AICc values are computed for the fitted model as a whole, and changing d or D changes the model structure; however, AICc is still defined for models with different differencing, but direct comparability can be problematic if the models are not nested or if over-differencing occurs. Practically, one should compare AICc of the two models with the understanding that differencing is part of the model and that AICc is still usable for comparison, though one should be cautious about over-differencing. Overall, the claim that they have the same amount of differencing is not guaranteed; the more accurate stance is that they may have different amounts of differencing depending on auto.arima’s selection. So the option asserting they have the same amount of differencing is not generally correct; the alternative stating they have different amounts of differencing is plausible and commonly observed.

Option 2: "The accuracy measures (RMSE, MAE, MASE) for the training set are lower for the [Select] ARIMA(0,1,0)X(0,1,1)12 model vs auto.arima model, which indicates that this model has better fit to the training set." 
- If the ARIMA(0,1,0)X(0,1,1)12 model yields lower RMSE/MAE/MASE on the training data, that indicates a closer fit to the observed training values under those metrics. However, a lower training error does not automatically imply better forecasting performance or generalization. It could reflect overfitting, especially if it is a more complex or more flexible seasonal structure that captures noise rather than underlying signal. The auto.arima model, designed to balance fit and parsimony (and often validated via information criteria like AICc), might produce a model with slightly higher training errors but better out-of-sample performance. Therefore, while the statement could be true in a strictly measured sense, it may be misleading if we interpret it as indicating superior predictive capability on unseen data. The key nuance is that training-set superiority does not guarantee better test-set forecasts; pay attention to cross-validation or a held-out test set for forecasting claims.

Option 3: "The accuracy measures for the test set (RMSE, MAE, MASE) are lower for the [Select] ARIMA(0,1,0)X(0,1,1)12 model vs auto.arima model, which indicates that this model does better at forecasting." 
- If, on a test set, the ARIMA(0,1,0)X(0,1,1)12 model shows lower RMSE/MAE/MASE than the auto.arima model, that would suggest better predictive accuracy on unseen data for that particular split. However, several caveats apply: the test set needs to be representative in time and size; the evaluation should be repeated across multiple forecasting horizons if possible; and the difference should be statistically meaningful rather than due to random variation. Additionally, auto.arima might have selected a model that is more robust to overfitting or that captures the data-generating process more faithfully over longer horizons. Therefore, a lower test-set error for the ARIMA model would support its forecasting advantage in that specific evaluation, but one should confirm consistency across horizons and folds before concluding universal superiority.

Option 4: (Implicit alternative for Option 3) "The accuracy measures for the test set are lower for the auto.arima model, which indicates that this model does better at forecasting." 
- The mirror scenario would be that the auto.arima model achieves lower test-set errors, signaling better out-of-sample predictive performance in the evaluated context. This outcome would align with the purpose of auto.arima, which is to select a parsimonious yet well-fitting model typically aimed at generalization. If this is observed, it would suggest the auto_arima model generalizes better on the test data, potentially due to more appropriate differencing, order selection, or a balance of nonseasonal and seasonal terms. As with the previous option, the reliability of this conclusion depends on the test design, sample size, and whether results are stable across alternative datasets or forecasting horizons.

Key takeaway across options:
- Differing amounts of differencing between the two models are common; auto.arima may select different d and D values depending on the data.
- In-sample (training) fit can be better for a model that overfits, but this does not guarantee superior forecasting performance.
- Out-of-sample (test) performance is the more robust measure of forecasting ability, yet it should be interpreted with attention to evaluation design and statistical significance.

If you need, I can illustrate with a concrete hypothetical example showing how d and D choices affect AICc, and how training vs. test RMSE/MAE/MASE trends might diverge between a manually specified ARIMA and an auto.arima-selected model.

BABS_V 502 BA1 BA2 2024W2 Assignment 04 - Due THURSDAY Feb. 6 at 11:59pm

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