Planning and scheduling as we know it today has been around since the early 1900’s. Henry Gantt introduced the first bar chart in 1910, and in the late 1950’s, Kelley and Walker developed the Critical Path Method (CPM). In 2008, a new scheduling algorithm known as the Graphical Path Method™ (GPM) was introduced by Dr. Gui Ponce de Leon. CPM and GPM share some important things in common, but there are also some fundamental differences between the two scheduling methods. GPM introduces a number of concepts previously nonexistent in CPM, and once these concepts are understood, they can yield significant advantages and efficiencies to project management professionals and non-professionals alike.
The most fundamental differences between CPM and GPM are described in summary below. Understanding these core differences forms the basis for getting started with GPM and quickly improving your planning and scheduling outcomes.
Activity placement: early and late dates vs. planned dates
In CPM, all activities are by default confined to their early dates. In order to place activities not on early dates, constraints must be used. Unfortunately, constraining activities ‘chokes’ their total float values. In GPM however, activities can be placed anywhere between their early and late dates, with no impact on their total floats. This is accomplished through the introduction of two simple but revolutionary concepts: drift and float, the sum of which is equivalent to CPM total float.
Float calculations: early and late dates vs. link gaps
In order to calculate total float, CPM executes a forward and backward pass of the schedule network and examines early and late dates. For any activity, total float is calculated as the difference between these dates. In GPM however, drifts, floats, and total floats are not a function of early and late dates. Rather, they are calculated from the time-units embodied in links, which connect activities and form relationships, and are network objects themselves with their own distinct properties. This groundbreaking concept is known as link-gap, and it is calculated as the difference between a predecessor’s end node and a successor’s start node.
Database vs. objectbase
Inherent to CPM-based software is a scheduling engine that continually processes a database in order to reflect schedule revisions and to generate an updated network model. Unfortunately, the disconnect between the database and the graphic model produces a cumbersome and opaque result. With GPM however, all revisions to a schedule are made directly to the network graphic, and the results are instantaneously reflected visually, and in real time. This is because schedule data is encapsulated directly with corresponding objects on the scheduling canvas. There are no sequential, batched, black-box calculations that are inherent to the CPM algorithm. The result is a transparent and intuitive system that promotes a closer connection between the planner and their schedule.
For a thorough comparison of GPM and CPM, and to learn additional best practices in making the transition, download the CPM to GPM Bridge presentation.
Note: this publication will be available for download soon!
Watch the webinar on pmi.org.