Percentiles

When a probabilistic simulation is complete, you are left with three forecasts based on their percentile. Specifically, there will be a P10, P50, and P90 forecast. These forecasts are automatically exported to the Analysis Manager, where they can be viewed from other worksheets, such as forecast or decline worksheets. When a probabilistic simulation is re-run, these forecasts update with the most recent results. In order to save a given probabilistic forecast such that it isn’t overwritten by the next simulation, simply rename the forecast.

The three lines that are visible on the rate vs time, and rate vs cum plots are the P10, P50, and P90 curves. These curves are defined as follows:

1. P90 — For each time interval, the P90 point is determined as the value for which 90% of the data points are higher. The production volume for each period is based on the calculated rate, and the cumulative production track is calculated by summing the calculated volumes. A line is drawn through each P90 point to obtain the curve. The P90 curve is often considered as a conservative estimate of reserves.

2. P50 — For each time interval, the P50 point is determined as the value for which 50% of the data points are higher. The production volume for each period is based on the calculated rate, and the cumulative production track is calculated by summing the calculated volumes.

3. P10 — For each time interval, the P10 point is determined as the value for which 10% of the data points are higher. The production volume for each period is based on the calculated rate, and the cumulative production track is calculated by summing the calculated volumes. The P10 curve can be considered a highly optimistic prediction.

Note:   In contrast to the former usage of percentiles in IHS CBM, the P10 and P90 parameters in Harmony are not calculated according to "proper" statistics theory. In other words, P10 no longer refers to the 10th percentile or the smaller, more conservative, estimate. In Harmony, P10 is defined by the value you will obtain 10% of the time or less; it is the 90th percentile. This value is therefore larger, or more optimistic. This change has been made as Harmony is frequently used in reserves work, and this is how reserves treat probabilities.

Similar calculations are performed for the other curves (P1, P25, P75, and P99), which are not shown as explicit lines on the plots. Instead, they are used to create the shading that you see between the lines.