In the Six Period Summary report, the Critical Path Length Index (CPLI) shows up in the report (at level 1 only) if there is schedule data for the period and the CpliUID field of the Project table is not null. For the task identified in the CpliUID field, we calculate the CPLI as (TotalFloat + Remaining Duration) / Remaining Duration.
(You can view or change the CpliUID field in the Project table by using the command Admin > Download Data File and selecting 'Projects (for selected Contract, Period)' in the Data Type list box, and selecting the appropriate contract and period in their list boxes above.)