During project execution, each attribute of the schedule (start, finish, duration) has two or more values. The Planned values are the Estimated values created during the planning process and copied to a baseline. A Baseline is a snap shot of the project schedule that establishes the “planned schedule” for the project. The Baseline fields do not change from this point forward. As tasks are started and finished the Actual dates for these events are entered in the original estimated fields for each activity. Actual Start and Finish values that differ from the Planned values will impact the estimated start and finish dates downstream based on their dependencies.
This concept can be confusing. A good way to visualize the schedule data lifecycle is to explore the way a global positioning service (GPS) operates during a trip.
Prior to starting a trip, a driver enters the starting and ending points into the system. The system then calculates the recommended path or paths based on known data. Upon selection of the desired path, the driver instructs the GPS to begin guiding the journey. The GPS then calculates the estimated distance (duration) and the expected travel time (elapsed time) and the projected arrival time (project finish date). The path is saved into the GPS memory (baselined) and is used as the “planned route” from that point forward.
As the vehicle makes progress, the GPS informs the driver each time a new action is required such a turning, exiting, or navigating an interchange (initiation of new work packages). The GPS continually tracks the progress made (actual values) and uses this data to create new estimated durations, elapsed time and expected arrival time. If the driver makes faster than planned progress theses values are reduced and if the progress is slower than planned, the values are increased (revised estimated values).
Like the GPS, the PM monitors progress by using reportable status attributes in addition to the Actual Start and Finish Dates. These attributes provide indicators on the progress of an activity between its start and finish dates. There are many ways to collect and report activity performance. The two most common are tracking the percentage of work completed and the status of the activity (not started, started, completed). These two attributes provide the PM a quick indication of current activity and progress.