This excerpt taken from the PCG 10-K filed Feb 18, 2005.
UCLs, LCLs, UWLs and LWLs for Index 3 when Number of Lines is less than or equal to 125 and greater than or equal to six.
The UCLs, LCLs, UWLs, and LWLs for the control charts for each voltage class shall be based on exact binomial probabilities for those voltage classes having equal to or more than six but less than or equal to 125 median transmission lines per year.
A customized macro and a statistical software package approved by the ISO creates the proportion control charts. The macro determines the control limits and use of the exact binomial or the normal approximation to the binomial for computing the control chart limits. This macro ensures the UCL and LCL contains about 99.5% and the UWL and LWL contains about 95% of the binomial distribution. The percentile values of the UCL, UWL, LWL, and LCL are respectively 99.75%, 97.5%, 2.5%, and 0.25%.
The UCL, UWL, LWL, and LCL are calculated using the following formulas:
UCL = (X1+ (P2 - P1)/(P3 - P1))/ n
UWL = (X1+ (P2 - P1)/(P3 - P1))/ n
LWL = (X1+ (P2 - P1)/(P3 - P1))/ n
LCL = (X1+ (P2 - P1)/(P3 - P1))/ n
More information on the calculations of the proportion control chart limits is in the current ISO Transmission Facility Availability Performance Monitoring System Handbook.