The collapsed as-built method (also known as the "but-for" analysis) starts with the as-built programme and removes employer-responsible delay events to show what the completion date would have been "but for" those delays.
This retrospective method is useful when limited contemporaneous schedule updates exist. It works backwards from the actual completion date, stripping out the identified delay events and re-calculating the critical path to determine the extent of excusable delay.
The main criticism of the collapsed as-built method is that it requires subjective judgment about which activities to remove and how the remaining programme would have proceeded. Despite this, it remains widely used and accepted in many jurisdictions where better records are not available.