Time Series Analysis ARIMA Models 3

Question 1

What does 'invertibility' mean in the context of an MA model?

Accepted Answer

The MA model can be expressed as an infinite AR model

Answer

An invertible MA model has roots outside the unit circle and can be rewritten as an infinite-order AR process.

Question 2

Over-differencing a time series typically introduces which problem?

Accepted Answer

Negative autocorrelation at lag 1

Answer

Over-differencing adds unnecessary variance and often creates a strong negative autocorrelation at the first lag.

Question 3

What is a unit root in a time series?

Accepted Answer

A characteristic root equal to 1, indicating non-stationarity

Answer

A unit root means the characteristic equation has a solution of exactly 1, which causes the series to be non-stationary with a stochastic trend.

Question 4

In ARIMA modeling, what does 'd=2' imply?

Accepted Answer

The series was differenced twice to achieve stationarity

Answer

d=2 means second-order differencing was applied, removing both linear and quadratic trends from the series.

Question 5

Which metric penalizes model complexity more heavily than AIC?

Accepted Answer

BIC (Bayesian Information Criterion)

Answer

BIC applies a stronger penalty for additional parameters than AIC, especially with larger sample sizes.

Time Series Analysis Practice Test