Where is the critical path? This is one of the questions most often asked concerning schedule analysis. For some types of projects it is easy to anticipate where the critical path should be; for instance in high-rise building construction, the critical path is most likely to be: excavation, foundation, structure, weather enclosure (building envelope), MEP, finish activities.
Some activities will necessarily overlap. If the planned or actual critical path varies significantly from this most likely sequence of activities, then either some unique, one-of-a-kind activities are included in the work or something has disrupted the normal progression of the work. Sometimes when analyzing construction schedules MDC® finds parallel critical paths through various activities. This is not unusual and in examining the planned and actual critical paths for the work, the reasons for the parallel activities are typically easily determined.
It is the case, sometimes, that the schedule for the work is manipulated to present a “rosier picture” which can provide a false optimism (consider this ‘softer’ alternative) concerning the actual progress of either the main contractor or specialty subcontractor’s work production. In the mid-1980s, with the promise of on-site CPM schedule development and monitoring it was expected that disputes concerning project delays would become a thing of the past. Surprisingly, given the introduction of more powerful, PC-based project management software, as additional information and processing power came into the scheduling arena, additional complexity also found its way into typical project schedules.
The new scheduling tools allowed for rapid input of planned and actual activities resulting in larger and more complex schedules. The introduction of resource leveling techniques, coupled with the denser schedules greatly complicated the analysis work required to understand, debug and simplify the schedules into a more understandable presentation. So instead of having more insight into the activities comprising the critical path and allowing for more informed project management decision-making, the now ‘data rich’ schedules often disguised the most important information and clouded the insight that should have been apparent in a weekly or monthly schedule review. We were in effect ‘blinded by science.’
CPM scheduling has always included as much art as science and, depending on the project completion philosophy utilized by the owner/construction manager, the resulting schedules may contain more fiction than fact. It is worth observing that the typical time-based CPM schedule reduces the three-dimensional geometric problems of construction to a simple two-dimensional graphical representation. It is more accurate to envision the modern construction project as a four-dimensional schedule with time being the additional fourth axis. Each of the activities depicted on the schedule require inputs along another dimension to install and operate the completed work. Accordingly the attempts to tightly constrain and schedule construction activities is, at best, likely to be only a close approximation of the actual work and inputs required to complete the installation and operation of the systems for the project.
So returning to the question of whether the critical path in the CPM schedule is real or imaginary depends on where in the project lifecycle the actual work is at any point in time. At the beginning of the project the critical path is surely imaginary because it simply a projection of what’s anticipated to occur. If what was anticipated was always the case of what happens, then all projects would complete on schedule. However in the real world of construction projects all of the assumptions utilized in estimating and planning the project come under close scrutiny and are likely to change over the course of the work. In MDC’s schedule analysis experience we have often found that unrelated environmental factors outside the project control come to bear, significantly altering the planned sequence of activities.
In one significant project an earthquake occurring in Japan delayed and disrupted the project activities due to a worldwide shortage of structural steel as a result of the reconstruction in Japan following the earthquake. At no time following the event was it clear how or where the correct structural steel shapes would be found and shipped to the site. On this project, all projections of the pace and sequence of the steel work proved to be inaccurate disrupting all other related activities. Only clairvoyance could have allowed the owner, the engineers or the contractors to minimize the impacts of this environmental effect. Similar material supply disruptions occurred in the US after Hurricane Katrina – the rebuild effort sucked up plywood, sheathing and roofing materials throughout the southern U.S., adversely affecting projects throughout the United States.
Another schedule element of growing impact is commissioning. Commissioning, along with the scheduled completion of building control systems, is a highly variable set of interrelated tasks whose execution is often dependent upon almost every trade – you can’t fully commission an unfinished building.
After-the-fact evaluations of the planned and actual critical path activities can show wide variations between the expected and the real critical paths and longest path for the work. Not unlike ‘chaos theory,’ things that early on were considered secondary or even tertiary, can loom large and be a boulder in the path to completion. Accordingly, in developing and evaluating CPM schedules, art and science must both be brought to bear on the question and a full measure of relevant project experience utilized to arrive at a logical and likely CPM analysis.
MDC Systems® is a project and construction management consulting organization and has been intimately involved in CPM schedule development, analysis and expert witness testimony over the last 50 years.