A warehouse forecasts seasonal monthly demand using Holt-Winters. During testing, the predictions reproduce the broad annual seasonal pattern but consistently underestimate demand during the final three months.

The analyst proposes the following actions.

I. Examine whether the demand level or trend changed during the test period.

II. Immediately increase γ to its maximum value, without further analysis.

III. Compare residuals across time to identify systematic bias.

IV. Confirm that seasonal periodicity was specified correctly.

Which combination is the most defensible?