Industry: Construction and Infrastructure Product: Tamara Application: Construction schedule risk analysis
An 18-storey commercial tower was scheduled with a rolled-up critical path of 865 working days — a clean July 2028 handover the developer took to its anchor tenant and its lender. The schedule was built the orthodox way: most-likely durations for each of eleven activities, chained through the network, read off the end date. The number was not wrong on its own terms. It was simply the answer to a question nobody should ask of a project schedule — what happens if every activity lands exactly on its most-likely duration and nothing goes wrong?
Rebuild the same network in Tamara, Vose Software's Monte Carlo project risk tool, with each activity carrying a Beta-PERT duration and six discrete risk events layered on top, and the deterministic date dissolves into a distribution: P50 December 2028, P90 April 2029, and a probability of finishing by the published July 2028 plan of just 1%. The S-curve below is the whole argument — the deterministic plan sits at the very foot of the curve, in the regime a project commits to only if it intends to re-forecast every quarter.
The gap between the July 2028 plan and the P80 finish is 232 days — roughly 7.6 months. That gap is not pessimism; it is the project's true schedule contingency, and it was invisible to the deterministic roll-up.
A bar-chart critical path adds most-likely durations along one assumed-longest chain. Two things break it. First, durations are right-skewed — an activity can finish a little early but can overrun a lot, so the mean finish sits later than the most-likely roll-up and the mean simulated finish is 1,029 days against the 865-day plan. Second, with parallel chains (envelope, MEP rough-in and core all run off the structural frame), whichever chain happens to be longest in a given iteration drives the finish — so the project inherits the worst of several paths, not the average of one. The deterministic method sees neither effect.
Tamara reports each activity twice: its criticality index (how often it lands on the critical path) and its cruciality (the rank-correlation between its duration and the project finish — i.e. how much its variability actually moves the end date).
Interior fit-out dominates (cruciality 0.56), followed by the structural frame (0.51) and permits & mobilisation (0.38). This is the action list: the finishing chain (interior fit-out → MEP commissioning → inspections → handover) is on the critical path in essentially 100% of iterations, so the highest-leverage week of risk reduction is buying duration certainty on fit-out and the frame — not on whichever task the team happens to find easiest to compress.
The same simulation places every activity in time as a band, not a bar:
The whiskers widen downstream because uncertainty compounds along the chain: each activity inherits the spread of everything it waits on, so handover carries the accumulated variability of all ten activities before it.
Beyond continuous duration uncertainty, six discrete events were modelled as Bernoulli risks — each may or may not occur, but if it does it adds delay (and cost). Ranking them by expected schedule impact (probability × delay) gives a clean Pareto:
Five of the six events carry ~80% of the expected discrete-event delay — severe-weather lost days (3.4 weeks expected), skilled-labour shortage (2.9), permit delay (2.6), structural-steel supply delay (2.5) and a major design change (2.2). Once the ranking is visible, the risk-response budget writes itself.
Every day beyond the deterministic plan carries extended general-conditions and escalation overhead, so the schedule distribution drives the cost distribution. Tamara let the team price two mitigations together — weather protection plus an early structural-steel supply contract, and a fit-out labour pre-commitment — and compare before/after on the same axis:
Without mitigation the cost distribution runs to a mean of $99M and a P90 of $108M, with a 20% probability of breaching the $105M budget. The $2M mitigation package cuts the P80 finish by 66 days (1,097 → 1,031 days), and because schedule drives cost, the overrun probability falls from 20% to 7% — the mitigation pays for itself several times over in avoided overhead, not in the line items it touches directly.
A construction schedule is not a date; it is a distribution with a body the team can plan against and a tail it cannot afford to ignore. Tamara is what turns "when will it finish?" into a probability the developer, the tenant and the lender can all sign.