What a base period is
Every escalation clause needs a starting point: the index value everything else gets measured against. That starting point is the base period. Every future adjustment is a comparison between the current index value and this one fixed number.
Why the wrong choice breaks the whole contract
If the base period lands on an unusually high or low index value, every single future adjustment inherits that distortion. A base period picked during a seasonal spike will make every later adjustment look smaller than it should. A base period picked during a temporary crash will make every later adjustment look larger than it should. This error does not fade over time — it stays baked into the contract until it ends.
The usual choices
Most contracts use either the contract signing date or the bid submission date as the base period, because both are real, documented events that are hard to dispute later. Avoid picking a date just because it produces a favourable-looking base value — that invites a challenge from the other side.
Watch for seasonal and volatile months
Some indices move in a predictable seasonal pattern — construction materials often price higher in summer, energy indices move with weather. Check whether the index has a seasonal pattern before locking in a base period, and prefer an averaged base period (three months, not one) if the index is volatile month to month.
The decision that actually matters
Tie the base period to a real, documented contract event, and check the index's recent history before locking it in. A base period chosen for convenience, without checking whether that specific month was typical, is the single most common way an escalation clause goes wrong.