Critical path barriers obscure and invalidate the true critical path through the schedule. Does your schedule accurately represent the longest path or critical path through the schedule network?
Critical path is an important concept in scheduling. It determines the duration of the project. It also highlights where to investigate for schedule optimization efforts; shortening the activity durations of critical path activities compresses the schedule. A valid critical path is important for the quality of a schedule.
Miscalculation of a critical path may cause activities that are not critical to appear as though they were critical. An invalid critical path also hinders reallocation of resources from noncritical activities to critical activities.
This article discusses issues that prevent the critical path from being the longest path through the network.
Critical path barriers obscure and invalidate the true critical path through the schedule. Project managers, therefore, need to be aware of these roadblocks that may hinder their critical path definition. The following are common critical path barriers.
Discontinuities are a major hindrance to a proper critical path. The critical path must be continuous from the start date or status date to the project completion milestone. There should be no breaks or unaccounted time. The critical path may branch off into other activity sequences, but they must all converge at the finish milestone. Critical path breaks must be reviewed and properly addressed.
2. Number of Critical Activities
It is not helpful to predetermine the number of activities in a schedule that should be critical. The number of activities along the critical path should correlate to the required schedule visibility to manage the project and reduce risk. It should be noted that if the ratio of critical path activities to the total remaining activity count is 100 percent, the project may be overly resource constrained. A critical path that has only a few activities may represent support effort, such as Level Of Effort (LOE) activities, and not the true project driving activities that produce deliverables.
3. Logic Sequencing
Float calculations are directly affected by the logical sequencing of events. Critical activities are identified by their float values, and, therefore, are directly related to the logical sequencing of events and float calculations. Activities missing dependencies, linked incorrectly, or performed out of sequence will miscalculate float. And the critical path computes incorrectly.
4. Date Constraints
Broad usage of date constraints in the planning stage of a schedule is acceptable. They hold the need dates in place. A respective constraint renders the activity critical, and any activity that overruns that date registers with negative float. The scheduler must adjust logic or make other changes to meet the key dates, and remove negative float. If need dates are feasible the negative float free schedule supports the key dates.
So temporary use of hard constraints helps verify the ability of the schedule to meet need dates. However, insertion of date constraints simply to fix an activity in time convolutes critical path calculations. And each date constraint in the schedule generates its own sequence of critical activities, which defeats the Critical Path Method (CPM) analysis.
Lags are discouraged because they obscure the identification and management of critical activities. Lags are often misused as target date tools to make the activity commence on a preferred calendar date. A lag simply represents the passage of time, and they do not have resource assignments. This makes it difficult to evaluate a critical path that has lags, because it is not clearly specified whether or not the lag represents work done by a resource.
Lags do not appear in critical activity reports, because they are not activities. Thus, critical activity reports could result in misguided efforts to shorten driving predecessors. The preferred approach to lag usage is to model the effort as an activity, perhaps, with assigned resources.
It is important that the critical path truly represent the longest path through the schedule network. Discontinuities along the critical path are a showstopper and must be addressed. Too many critical activities indicates a resource constrained project. Too little may imply level of effort support activities are included in the critical path.
Date constraints during project planning are helpful, but should be avoided in the final schedule. Date constraints obfuscate calculations, and may result in multiple critical paths, which is not helpful. The critical path should be free of lags to support the identification and management of critical activities. Lags are difficult to evaluate because the effort they represent is not clear. Consider replacing lag with an actual activity.