One method analyses what actually happened in successive periods; the other models the predicted effect of an event. The choice shapes the answer and the cost of getting it.
Delay analysis methods differ in whether they look forward from the event or backward from the record. A time impact analysis models the effect of an event on the programme as it stood at the time. A windows analysis divides the project into periods and examines what actually drove completion in each. Both are legitimate, they can produce different answers on the same facts, and the method should be chosen before the result is known.
Time impact analysis at a glance
The programme as it stood immediately before the event is taken, the event is inserted as a fragment of activities with logic, and the programme is recalculated to show the predicted effect on completion. It is prospective in nature, which makes it the natural method for assessing an extension of time application while the project is running. It depends entirely on the quality of the programme used, because a programme with weak logic will not transmit the effect correctly.
As built windows analysis at a glance
The project is divided into periods, usually bounded by programme updates or significant events, and the actual progress in each window is examined to establish what was genuinely controlling completion during that period. It is retrospective and evidence led, which makes it robust where the facts are messy and the programme updates are imperfect. It is also slower, more expensive and dependent on the quality of the as built record.
Direction of analysis
Time impact analysis: Prospective. It asks what effect the event was predicted to have at the moment it occurred.
As built windows analysis: Retrospective. It asks what actually controlled completion during each period, using the record of what happened.
Dependence on programme quality
Time impact analysis: Very high. The method only works if the programme has sound logic, realistic durations and a genuine critical path.
As built windows analysis: Moderate. A weak programme can be supplemented by as built evidence, though some programme basis is still needed.
Dependence on site records
Time impact analysis: Moderate. Progress data is needed to establish the status at the time of the event.
As built windows analysis: Very high. The whole method rests on knowing what was actually happening, so diaries and allocation records become central.
Treatment of concurrent causes
Time impact analysis: Weak. Modelling one event in isolation tends to understate or ignore what else was happening at the same time.
As built windows analysis: Strong. Examining each window naturally reveals whether more than one cause was driving completion.
Cost and time to perform
Time impact analysis: Lower. A competent planner can perform it on a defined event in a matter of days.
As built windows analysis: Higher. It requires the as built record to be reconstructed period by period, often over weeks.
How it is received in a formal dispute
Time impact analysis: Accepted, but open to the criticism that it models a prediction rather than reality, particularly where the predicted effect did not occur.
As built windows analysis: Generally well received where the records support it, because it describes what happened rather than what was forecast.
Usefulness while the project is still running
Time impact analysis: High. It supports a decision in time to change the remaining programme.
As built windows analysis: Low. It needs the period to have passed before it can be analysed.
When time impact analysis wins
- Assessing an extension of time application contemporaneously, before the outcome is known.
- Projects with a well constructed, properly updated programme and regular progress data.
- Single, clearly identifiable events with a definable start and a definable effect.
- Contracts that require the effect to be assessed when the event occurs rather than at completion.
- Situations where the parties want a decision quickly enough to influence the remaining works.
When as built windows analysis wins
- Retrospective disputes where multiple events overlap across a long period.
- Projects where the programme updates were poor but the site records are good.
- Cases involving concurrent causes, where the question is what actually drove completion.
- Final account and formal dispute settings where a tribunal wants to see what happened rather than what was predicted.
- Long contracts where the critical path moved several times.
How to decide
Use a time impact approach for contemporaneous assessment during the works, because a decision made in time is worth more than a perfect one made at the end. Use a windows analysis for retrospective disputes, for overlapping events and wherever the question is what actually drove completion. Where the contract specifies a method or requires assessment at the time of the event, that requirement comes first, and a method chosen to suit a desired answer will be identified as such.
Decide the method in writing before the analysis begins, and record why it was chosen and what data it will use. Then test the result against the site records: if the analysis says completion was delayed by a period in which the diaries show the critical trade working normally, the analysis is wrong whatever the programme says. The standards and practice notes that guide this work are revised from time to time, so confirm the current edition with your analyst, and take advice from a licensed adviser before relying on any analysis in a formal dispute.
Domira builds and supervises projects across Saudi Arabia, and the commercial discipline above is how its projects stay inside their budget and their programme. Send the drawings, the contract or the site and Domira will tell you where the risk actually sits.
Frequently Asked Questions
What is the difference between time impact analysis and windows analysis?
Time impact analysis is prospective: it inserts the event into the programme as it stood at the time and calculates the predicted effect on completion. Windows analysis is retrospective: it divides the project into periods and examines what actually controlled completion in each. The first supports a decision during the works; the second is usually stronger in a retrospective dispute.
Which delay analysis method is most accepted?
There is no single method that is always correct. What matters is that the method suits the available evidence, is applied consistently, is explained transparently and answers the question the contract asks. A retrospective dispute with overlapping events and good site records usually favours an as built approach, while a contemporaneous assessment under a contract that requires one will favour a prospective method.
Do I need a delay expert for an extension of time claim?
Not always. A competent planner can perform a sound contemporaneous analysis on a single clear event. Independent expertise becomes valuable where events overlap, where the programme is poor, where large sums turn on the result or where the matter is heading to a formal dispute. Where the dispute is already formal, the choice of analyst should be discussed with your legal adviser.
Can the same facts produce two different delay answers?
Yes, and that is one reason delay disputes persist. Different methods, different baseline programmes and different treatments of float and concurrency can each move the answer. The practical response is to be explicit about the method and the assumptions, test the result against the site records, and be willing to explain why a different method would give a different figure.
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