What an averaging window is
An index publishes a new value every month. A clause has to decide how many of those months to use before applying the adjustment: the latest single value, a 3-month average, a 12-month average, or something else. That choice is the averaging window.
Why it changes the number
A single-month value reacts immediately to whatever happened that month, including a one-off spike or dip that reverses the next month. A longer average smooths that noise out, but it also reacts slower to a real, sustained change in the underlying cost. Two contracts reading the same index with different windows will calculate two different adjustments on the same day.
Short window, fast reaction
A 1-month or 3-month window moves quickly. This suits inputs that genuinely swing fast and where both sides want the price to track reality closely, even if that means more frequent, larger adjustments.
Long window, smooth reaction
A 12-month rolling average moves slowly and rarely swings hard in either direction. This suits contracts where both sides value predictability over precision, and would rather absorb a slow-moving average than chase every short-term spike.
The decision that actually matters
Match the window to how the underlying cost actually behaves, not to a default. An input that moves in genuine step-changes (a new tariff, a supply shock) is poorly served by a 12-month average that takes a year to catch up. An input that jitters month to month without a real trend is poorly served by a 1-month window that just amplifies noise.