When a project schedule has multiple activity calendars in use, the Total Float values for a string of activities on the same driven network path may be different. This may confuse someone reviewing the pdf of a schedule, even if the relationship lines are displayed. This can also occur when multiple calendars are used on schedules with resource driven activities.
Once the activity and resource calendars have been developed and the appropriate calendars have been assigned to each activity, the schedule will model the activity durations based on legitimate work days.
For more background on calendars, you may want to read the following related articles:
Once the activity relationships have been assigned, the schedule will model activity network paths based on the activity durations and activity relationships resulting in a Critical Path(s), near critical paths, and the remaining work paths for the project, at that point in time, the Data Date.
The difference in Total Float values, for activities on the same driven path, is a product of the forward and backward calculations and the differences in the work days for activities on these paths.
(This difference in Total Float values is why we generally don’t define the Critical Path as Total Float = 0. But this is a subject for future articles.)
So, while it is best to develop and utilize multiple calendars to accurately project work, this practice also makes it difficult if not impossible to accurately use float values to determine which activities are driving on a path, when reviewing a pdf Gantt chart.
If an installation activity is driven by the fabrication/delivery activity for this product and the fabrication/delivery activity is driven by the review and approval of the product data for this product which is driven by the development and submission of the product data, we can have a couple of different Total Float values for this group of activities.
The development and submission may have a 5-day work week with holidays calendar assigned. The review and approval activity may have a 7-day workweek calendar assigned because the submittal specification section requires 15 calendar days for submittal reviews. The fabrication/delivery activity may be assigned a 7-day calendar and the installation activity could be assigned a 5-day work week with holidays or 5-day work week with holidays and weather days calendar.
The Total Float values for the activities with a 7-day calendar assigned will be higher than the Total Float values for other activities because there are more work days available during the calendar day duration for the network path this group of activities falls on.
A reviewer may make the assumption that these activities are on the same path based on dates and because this is hard logic, they may be correct. But once we introduce preferential logic for trade management or phasing, things become much more difficult.
As you can see, reviewing a pdf Gantt Chart is not a simple matter. Assumptions can be made, but they are just that, assumptions. Only by reviewing the mpp or xer project file can a reviewer really understand what schedule settings and assignments are in place.
Plan your schedule….
Paul Epperson CCM, PMP, PSP, PMI-SP
Paul has extensive experience as a Construction Manager. Over time, he became convinced that there is a critical shortage of skilled planning and scheduling professionals in our industry. In 2009, he backed away from his work as a Construction Manager and began focusing on planning and scheduling. He now serves our industry as a subject matter expert in this area.