# Monte Carlo on Scenarios

Monte Carlo on Scenarios allows comparison of the probabilistic results of multiple scenarios.

PetroVR ensures compatibility between samples from scenarios. If a variable with distribution is shared by two or more scenarios, the same sampled value is used for all scenarios. Therefore, when the probabilistic results from a scenario differ from those produced by another, this can no doubt be attributed to intrinsic modeling differences, never to unsynchronized sampling.

The Monte Carlo on Scenarios screen includes a shutter pane containing the Monte Carlo Control Window for the base project.

When running Monte Carlo on Scenarios, only scenarios that have been modified since the last Monte Carlo run are simulated again. Scenarios that did not change are skipped (the Log Window informs you about this). Note also that modifying the base case invalidates all results and thus Monte Carlo has to be run again on every scenario.

Variables with a defined probability distribution must be the same in the base project and all its scenarios, although the definition itself can be different. This implies that you cannot define a distribution on a variable in a scenario, without defining it in the base project too (even if you just assign a constant distribution to it); conversely, you cannot clear a distribution in a scenario without clearing it in the base project.