P50 is the value with a 50% probability that the actual outcome will be at or below it. It is the median of the simulated results. A P50 finish date of 14 September 2028 means the project is as likely to finish before that date as after it. P50 is a coin-toss estimate, not a safe one.
S-curve. Illustrative QCRA result for a $120m base estimate. The highlighted point is the P50.
How P50 is calculated
A Monte Carlo simulation runs the schedule or cost model thousands of times. P50 is the middle result once all iterations are sorted: half the outcomes are lower, half are higher.
P50 = median of the simulated outcomes = the value x where P(outcome ≤ x) = 0.50
Worked example
A refinery revamp has a deterministic Primavera P6 finish date of 30 June 2028. After a quantitative schedule risk analysis (QSRA), the illustrative results are:
Measure
Finish date
Deterministic P6 date
30 Jun 2028 (about P18)
P50
14 Sep 2028
P80
20 Nov 2028
P90
5 Jan 2029
The P6 date sits at roughly 18% confidence. Even the P50 is 11 weeks later. This is the usual pattern: a deterministic plan assumes nothing goes wrong, so it lands well below the median.
P50 is not the mean
Cost and schedule outcomes are skewed to the right: things can overrun by far more than they can underrun. That pulls the mean (the average) above the median. In the cost example on the P90 page, the P50 is $126m while the mean is closer to $127m. When a report says "expected cost", check whether it means the mean or the P50.
When to use P50
Internal targets and forecasts that should be neither padded nor optimistic.
Portfolio planning, where overruns on some projects offset underruns on others.
Comparing options, where a like-for-like central estimate matters more than confidence.
Common mistakes
Committing externally at P50. Half of all outcomes miss it. External commitments are normally made at P80.
Calling the P50 the "most likely" value. The most likely value is the mode, which is usually lower than the P50 on a right-skewed curve.
Assuming P50 equals the base estimate. A base estimate without risk usually sits well below P50.
P50 is the median outcome of a risk simulation. There is a 50% chance the actual cost or duration will be at or below it and a 50% chance it will be higher.
Is P50 the same as the average?
No. P50 is the median. For right-skewed cost and schedule outcomes, the mean is usually higher than the P50.
Why is my deterministic schedule below P50?
A deterministic schedule assumes every activity goes to plan and no risk occurs. Once uncertainty and risk events are modelled, most plans land between P10 and P30.
Learn to build and defend these models
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