Delay analysis methods compared — which one should you use?
The choice is driven by the records you have, not by which method is best. Prospective methods (impacted as-planned, time impact analysis) model what a delay should have done. Retrospective methods (as-planned versus as-built, windows) examine what actually happened — and are harder to attack.
Prospective: modelling what should have happened
Impacted as-planned inserts the delay event into the original programme and sees what it does to the completion date. It is simple, cheap, and weak — because it takes no account of what actually happened on site, including the contractor’s own delays.
Time impact analysis is the more respectable version: it inserts the event into a programme updated to the moment just before the event, so it reflects reality up to that point. It is the method most contracts prescribe for prospective assessment of an ongoing claim, and it is what you use when the project is still running.
Retrospective: examining what did happen
As-planned versus as-built compares the programme with what was actually built, and identifies where the critical path shifted. Windows analysis does the same in slices — dividing the project into periods and asking what was driving completion in each.
These are harder to attack, because they rest on facts rather than on a model. And they are considerably more expensive, because they require an as-built record granular enough to reconstruct the critical path — which most projects do not have.
The honest selection rule
Method selection is usually presented as a technical judgement. In practice it is dictated by your records. A project with weekly quantity records, a maintained programme and a proper diary can run a windows analysis. A project without them cannot, and will end up with an impacted as-planned that the other side will take apart.
Which relocates the decision to where it actually belongs: two years earlier, on the day somebody decided whether to update the programme monthly and record installed quantities by location. The delay analysis you can run was chosen then, by people who had no idea they were choosing it.
In the guides
References
- SCL Delay and Disruption Protocol — the recognised delay-analysis methods and the records each requires
Zepth is the construction project delivery platform — it runs construction, procurement and asset management on one record, and does the work: reading the drawings, reviewing the submittals, matching the invoices and flagging the risks, with a human sign-off on anything consequential.
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