Time Impact Analysis: model the event, measure the move

What a Time Impact Analysis is

A Time Impact Analysis is a forward-looking schedule technique. You take the CPM programme as it stood the day before an event, add a small model of that event — a fragnet — and recalculate. The distance the completion date moves is the time impact, and that is the extension of time. It is done while the project is still live, so the adjustment can be agreed in near-real time, and it deals with time only — the cost follows once the days are settled. This is the method set out in AACE Recommended Practice 52R-06.

The method, in eight steps

1. Model the event as a fragnet — the fewest activities and links that capture it.
2. Select the unimpacted schedule — the last reviewed update statused just before the event (or the baseline, if the delay pre-dates the first update).
3. Insert the fragnet at zero duration and recalculate: the dates must still match the unimpacted schedule. If they don’t, the model is wrong — fix it before going on.
4. (Optional) Where the delay is long or mitigation has already happened, adjust for it so the calculated impact tracks the actual.
5. Insert the real durations and recalculate the CPM.
6. Identify the target activity that measures the impact — usually substantial completion.
7. Determine the total time impact, in work days or calendar days as the contract requires.
8. Eliminate delay dates already awarded — no single day is counted twice.

Why “prospective” matters

Prospective means it is a forecast, made during the works, precisely so the entitlement can be resolved before the impact is absorbed into the noise of the project. The longer you wait, the less valid it becomes — and a drawn-out approval can itself breed a constructive acceleration claim. Its after-the-fact cousin, the retrospective or forensic TIA (AACE 29R-03), is what you reach for once the delay has already run.

When it’s most reliable

A prospective TIA is strongest when the work plan is frozen, the event is forward-looking, and the delay is short — under one reporting period. It weakens when the schedule update no longer reflects site conditions, the work is non-serial and easily re-deployed, or heavy mitigation has already been done — in those cases a forensic analysis is the honest tool.

A TIA isn’t a re-run of history — it’s a disciplined forecast: model the event as a small fragnet, prove the model is clean at zero duration, then let the critical path show how far completion moves.