Delay Analysis Methods
But-For Delay
But-for delay analysis reconstructs a project without one specific delay event to isolate how much later the project finished because of that event alone.
Definition
To measure a single event's true impact, a but-for analysis removes that event from the schedule (or the as-built record) and recalculates what the project's completion date would have been without it -- the "but-for" date. The gap between that hypothetical date and the actual completion date is the delay attributable to the removed event. It's closely related to Collapsed As-Built, and the two terms are sometimes used loosely to describe similar techniques, though But-For is typically framed around isolating one disputed event specifically, while CAB more broadly reconstructs the full as-built network with one or more events removed.
Example
Removing a 40-day owner-directed specification change from a project's schedule and recalculating shows a hypothetical completion 40 days earlier than what actually happened -- since nothing else in the schedule had spare float to absorb any of that time.