Schedule Risk Analyzer

Why the schedule’s own date is usually optimistic

A schedule is built from single numbers: this activity takes 20 days. Everyone knows it might take 18 or 30. What most people do not see is that the finish date inherits the bad luck and not the good. Where several paths of work meet, the work after the meeting point waits for whichever path arrives last, and the chance that at least one of three paths runs late is much higher than the chance for any one of them. Risk analysts call it merge bias. It is why a schedule where every duration is honest still has a poor chance of its own date.

What the simulation does

Each run draws a duration for every activity from its range, adds the days of any risk that happens in that run, and works out the dates with the same critical path calculation as this site’s Critical Path Calculator: all four link types, lags, calendars and progress. Thousands of runs give a spread of finish dates for the project and every milestone. The page reads that spread three ways: the chance of any date, the date you can be 50% or 80% confident of, and which activities, when they run long, move the finish most.

The activities that matter are not always on the critical path

An activity with three days of float and a wide range, or a real chance of a setback, will sit on the path that sets the finish in a large share of runs. A schedule report shows it as having float to spare. This tool marks those activities, because they are where a schedule is most misleading and where attention pays off. The page also runs the same draws with only the critical path allowed to vary, and shows the difference: that gap is the merge bias in your schedule, in working days.

Where the ranges come from

From you. The tool cannot know that the vendor is slow or that testing always takes longer here. Set a default range for the whole schedule (low, medium or high uncertainty), then change the activities you know about, from a spreadsheet or on the page. A risk with a chance and a number of days, such as a 30% chance the vendor’s configuration needs another 12 days, is often more honest than stretching a range to cover it.

What this does not do

It does not tell you what date to promise; that depends on how much confidence the promise needs, which is a judgment for you and whoever you report to. It simulates durations and risks, not resources: two activities competing for one person are not leveled. Its dates follow this site’s critical path conventions, which can differ slightly from the software that wrote the file, and the page says how. The result is only as good as the ranges put in.