The Critical Path Length Index (CPLI) is a measure of the relative efficiency to complete a milestone on time. The CPLI is used to calculate and graphically display the longest, continuous sequence of tasks/activities through the network from contract start (or current status date) to contract completion and/or other major program milestone. The desired target efficiency ratio is “1.00” or as close as possible. If the efficiency ratio is at or over 1.00, the status is favorable; if it is less than 1.00, the status is unfavorable.

The CPLI requires determining the program schedule’s Critical Path Length (CPL) and the Total Float (TF). The CPL is the length in work days from time now until the next program milestone that is being measured. TF is the amount of days a project can be delayed before delaying the project completion date. TF can be negative, which reflects that the program is behind schedule. The mathematical calculation of total float is generally accepted to be the difference between the "late finish" date and the "early finish" date (late finish minus early finish equals total float). The formula for CPLI is as follows:

Critical Path Length Index (CPLI) = (CPL + TF) / CPL

Critical Path Importance: The CPLI is a measure of the relative efficiency required to complete a milestone on-time. A CPLI of 1.00 means the program must accomplish one day’s worth of work for every day that passes. A CPLI less than 1.00 means the program schedule is inefficient and the program will not achieve its baseline completion date without implementing corrective action . Likewise, a CPLI greater than 1.00 means that the program schedule is running efficiently in relationship to the milestone (i.e., finishing before the baseline date). The CPLI will usually be calculated to contract completion, but may be calculated for an interim milestone.

Example: Through April the supplier network schedule calculates a program critical path length of 720 work days (3 years) with 20 work days (1 month) of positive total float from contract award to completion:

CPLI = [( 720 + 20 ) / 720] = 1.03

At present, the supplier’s CPLI of greater than 1.00 indicates that the supplier may experience some delays, upto the total float value, and still achive the baseline project completion date.  The CPLI should be tracked throughout the contract period of perfomance to gauge the realism of completing the project on time.