Force Outage Rate (FOR) is computed using the following equation:
FOR = FOH/(FOH+SH)
Where;
Forced outage hours (FOH) is the number of hours a unit was in an unplanned outage state
Service hours (SH) is the number of hours a unit was in the in-service state. It does not include reserve shutdown hours.
One should be careful to note that the FOR is not a rate at all but rather an estimator for a probability.