This excerpt taken from the PCG 10-K filed Feb 18, 2005.
The calculation of the CLs for each of the three control charts is similar to the calculation of the annual Availability performance indices described in Section 4.3.1 except that the period for which data are to be included in the calculations is expanded from a single calendar year to the ten years, unless a shorter period is justified by the PTO, for the period immediately preceding the ISO Operations Date, or immediately preceding the date a TO becomes a PTO. To account for this change a count of Transmission Line Circuit years is included in the equations as shown below to enable derivation of CLs which represent average performance during a multi-year period.
CL for Annual Transmission Line Circuit Forced Outage(IMS) Frequency
CL for Annual Accumulated Forced Outage(IMS) Duration for those Transmission Line Circuits with Forced Outages(IMS)
CL for Annual Proportion of Transmission Line Circuits with No Forced Outages(IMS)