“Time passes, and little by little everything that we have spoken in falsehood becomes true.” -- Marcel Proust
There is an increasing and maturing understanding by the legal community, arbitrators and some courts of the fundamental technical principles underlying forensic schedule analysis. Some arbitrators and courts are now even using CPM scheduling terms such as “critical path” and “float” in their decisions when discussing after-the-fact delay analyses.
It is positive that more court decisions are providing guidance as to what makes a delay analysis credible. As an experienced delay analyst; however, I fear a little bit of knowledge may be a dangerous thing. Why? Because determining the time impact of a delay event is not as simple as looking at a schedule to see if that event affected the critical path, or is longer than the amount of float available to it.
The following are five things attorneys should know about float before using it to measure and argue the time impact of a delay event:
CPM scheduling software uses a mathematical model of the construction of a project to forecast completion dates for the work making up the project and remaining to be done after the “data date” of the schedule.
The creator of the CPM schedule develops it by dividing the project into work activities of fixed durations and linking these activities into a logical order (work paths) from the schedule’s data date to the completion of the project. Using this information, CPM scheduling software calculates completion dates for the work activities.
These work paths make up the CPM schedule’s “network” of work activities. The CPM schedule network of activities’ longest path (“critical path”) defines the shortest possible time for the completion of the project or other project milestones.
Float is often defined as the difference between the length of the longest path in the CPM schedule network and shorter work paths.
While there is usually a general order that work must be performed on most projects, CPM schedulers often have considerable flexibility in how they schedule the work. This is because there are usually many activities making up the project that do not have physical or safety characteristics that restrict the sequence in which they must take place.
Not only can most projects be constructed in many different sequences (“preferential logic”), the scheduler usually needs to make assumptions regarding the availability of labor and other resources needed to construct the project.
These critical resource assumptions dictate the durations of work activities and the number of work activities that can be worked on at a given time in a CPM schedule, which in turn affect float values in the CPM schedule.
The scheduler can also dictate when the contractor can or cannot work on certain activities (“constraints” and “calendars”) and can tell the CPM scheduling software how to handle out-of-sequence work or resource shortages when making its calculations.
Constraints, calendar, resource assumptions and CPM software calculation settings also affect the CPM schedule software’s calculation of projected early and late start and finish dates and float values of work activities within the schedule.
For these reasons, CPM schedules prepared by different schedulers and the float values in them are usually different. In other words, float is a characteristic of the CPM schedule not the project itself.
In addition to being specific to the CPM schedule, float values of specific work activities are also specific to the data date of the CPM schedule. A CPM schedule’s float values change over time based on progress before the data date and changes made to the schedule after the data date.
Float can be thought of as schedule contingency. Consumption of float does not extend the project duration until it is exhausted. However, like budget contingency, as float is consumed, it becomes more difficult to accommodate changes and unexpected circumstances in the future; thus, increasing the risk that the project may finish late.
Because float values for work activities change over time, it has long been recognized that one must evaluate the impact of delays using the CPM schedule in existence at the time of the delay.
As described above, CPM schedulers make many choices when they develop their schedules. These choices by the CPM scheduler directly affect whether the schedule shows a delay to be critical or non-critical and should be examined closely when using the schedules as a delay analysis tools.
Furthermore, critical delays are often predicted to extend past the CPM schedule’s data date and cause critical delays in the future. Delays forecasted after the schedule’s data date often do not actually happen due to future events taking them off the project’s critical path.
More important, the flexibility in most CPM schedules often makes it possible for the CPM schedule to be re-sequenced or changed to reflect added resources to create float and mitigate these “unrealized forecasted delays.”
CPM schedules are estimates that are only as good as the people that prepare them. CPM scheduling software assumes the schedule does not contain errors and the project can be built according the early start, early finish dates and the late start-late finish dates calculated by the software.
Most CPM schedulers check and validate the feasibility (sequencing and resource needs) of performing the work according the early dates forecasted by the schedule. In my experience, few do the same for the late start and finish dates in the schedule. When the late dates in the CPM schedule are not feasible, the resulting float values for affected work activities are wrong.
In other words, CPM schedules do not always speak the truth when it comes to float and the determination of critical delays.
Finally, did you ever wonder why the CPM schedules that you received as .pdfs during discovery do not show float values? That happens because the person that printed the schedule chose not to show them when he/she printed the schedule.
More important, it should be apparent from the above that one must know the many assumptions and calculation choices used by the CPM’s scheduling software to calculate forecasted work activity start/finish dates to fully understand the work activities’ float.
Usually the only way to know this information, which is critical to analyzing delays, is to obtain the schedule electronically in the native CPM scheduling format.
In closing, remember this: simply put, just because a computer-generated CPM schedule shows a work activity or delay is critical or has float, it does not necessarily make it so. Before relying on float values or any other information shown in a CPM schedule, you must understand the peculiarities of the CPM scheduling software and consider the choices made by the schedule creator.