By Barry Clark, PMP, PMI-SP
One the very heated and passionate debates of the project management discipline is that of the capabilities of the well-known critical path form of schedule analysis and management and the lesser-known critical chain. Guest columnist, Mr. Barry Clark, a credentialed schedule professional, writes this month’s Schedule Dispatch on the major differences between these two often-confused scheduling tools.
Critical chain (CC), derived from the Theory of Constraints, is the longest chain of tasks in the schedule that accounts for both duration and the resources required for tasks. Simply stated, the critical chain is the ‘resource constrained critical path’. By contrast, the Critical path (CP), the longest chain of tasks in the schedule that represents the shortest amount of time to project completion, is irrespective of resource availability or resource allocation or over-allocation. Critical path strictly considers task duration and float (the difference between late finish and early finish, or late start and early start), ignoring resource availability, over-allocation, and possible multiple tasks having assigned identical resources that are simultaneously scheduled. Resource over-allocation inevitably results in bottlenecks and/or scheduling conflicts that invariably lead to schedule delays.
One of the key differences between CC and CP is their use of buffers. Buffers are added to tasks or schedule as blocks of time to prevent slippage. Critical chain utilizes buffers, while Critical path does not. There are three types of buffers:
1) Feeding Buffers: time-block set at the end of a sequence of non-critical tasks;
2) Resource Buffers: time-block set aside to indicate resource needs. It is basically a resource place-holder;
3) Project Buffer: time-block set aside at the end of the project.
These buffers are strategically placed along the critical chain to account for and accommodate resource allocation, risk avoidance, and human behaviors, i.e. Parkinson’s Law and student syndrome. Parkinson’s Law is filling available time with redundant work when tasks are completed early (if you know you have a set time to complete a task, people generally will use the entire time); student syndrome is putting off until the last minute what should have been initiated earlier. Both issues are chronic problems in traditional scheduling using CP analysis.
Comparatively, the Critical path doesn’t consider resource allocations or dependencies, or the use of time-blocks (which reduces schedule risk), and basically ignores non-critical tasks, which can easily become critical. When projects inevitably run afoul, CP calls on schedule crashing and fast-tracking to ‘right-the-ship’ (move back to the left, or ‘left-the-ship’).
Given the nature of projects (and people who work them), including poorly defined project tasks, unknown pop-up tasks, unrealistic durations, multi-stakeholders with varying interests, volatile financial/ resource markets, etc., it is little wonder why projects run at an unacceptably high failure rate. Critical Chain analysis accounts for many known and unknowns (risks) and addresses the human factor, which consequently keeps projects under control more effectively.
With focus on resource leveling and bottle-neck avoidance (and thus risk avoidance), Critical chain scheduling paints a more realistic view of current status and future project outcome given that many projects are often underfunded and understaffed (usually due to Management’s demand to do more with less). If they are running a tight critical chain ship, the Project Manager need not sound the alarm to arms since the buffers have provided the needed relief; if however, they are running a Critical path ship and rough gales are eminent, crashing or fast tracking the schedule may exacerbate an already precarious situation.