OPS.10.2:4.4 - Derive time reserve and conditional criticality

First state the event against which reserve is measured. If the proposed final completion is C and its deadline is D, D-C is the plan’s final margin. A negative value shows that this plan misses the deadline; it does not by itself exclude another plan.

For a precedence network with fixed durations, continuous calendars and no binding resource contention, calculate early starts forward and latest starts backward. Start the backward pass from the selected final time T, retaining any earlier hard intermediate deadlines. For each successor, subtract the required intervening delay from that successor’s latest start. The earliest of those bounds and any deadline on the operation gives its latest finish; subtract its duration to obtain its latest start. Latest start minus early start is its total float relative to those targets. Free float is the delay that leaves the successors’ early starts unchanged, including their required intervening delays.

When only the final event governs this calculation, setting T to the earliest final completion identifies zero-float paths controlling that completion. An earlier intermediate deadline can instead give zero float to work protecting that intermediate event; identify which event the result concerns. Using a later final deadline adds delivery margin where other commitments permit it, so an operation on the longest precedence path can have positive deadline float. Several longest paths can exist. Serial operations may draw on the same margin, so their individual floats are not independent allowances to add together.

With shared resources, retain the selected allocations and orders when making the timing calculation. For whole-operation exclusive occupations, add the chosen resource-order relations to the precedence network; its timing is conditional on those orders. For partial occupations or calendars, use the corresponding phase and window constraints. A different allowed order or calendar can change the controlling sequence.

To assess a proposed postponement, hold the stated commitments fixed, delay that start, and refit the affected work. Identify the latest feasible placement or the set of feasible start windows that still meets the protected event. Calendar gaps can make intermediate start times infeasible even when a later window exists. A backward arithmetic pass that ignores those gaps supplies no usable postponement permission.

Call a sequence critical only with its plan, resource choices and target event recoverable. Test a claimed critical operation by changing its duration or availability and recalculating the consequence at that scope. An operating bottleneck concerns the mechanism limiting sustained flow under a workload; it need not be an operation controlling this finite deadline. OPS.9 supplies that diagnosis.