P50 (and P90, Mean, Expected and P10)
When probabilistic Monte Carlo type evaluations are adopted, this is a statistical confidence level for an estimate.
P50 is defined as 50% of estimates exceed the P50 estimate (and by definition, 50% of estimates are less than the P50 estimate). It is a good middle estimate. Mean and Expected (same level of measure just different names) usually lie about the P40-P30 levels in oil field evaluations and are therefore high estimates. P90 and P10 are low and high estimates respectively.
P90 means 90% of the estimates exceed the P90 estimate. It does not mean that the estimate has a 90% chance of occurring – that is a very different concept.
The central limit theorem indicates that the P50 estimate has more chance of occurring than the P90 and P10 estimates.