Planning Using Primavera Project Planner P3 Version 3.1 Revised 2006

A project schedule is usually updated at the end of a period, such as each day, week or month. One of the purposes of updating a schedule is to establish differences between the Baseline/Original Plan and the Current schedule.
The following information is required to status a schedule :
Activities commenced in the update period require:
Actual start date of the activity.
Remaining Duration from the status date or Expected finish date.
Costs and Hours or Quantities to date and/or Costs and Hours or Quantities this period or Total to date.
Costs and Hours or Quantities to complete.
Activities completed or started and completed, in the update period require:
Actual Start date of the activity.
Actual Finish date of the activity.
Costs, Hours and/or Quantities to date.
Activities not commenced require:
Changes in logic or date constraints.
Changes in Costs or Hours or Quantities.
Changes in duration.
Once this information is collected the schedule may be updated.