CPM (Critical Path Method) is a methodology that puts a strong focus on deliverables. The three main variables are time, cost and performance of the deliverables (e.g. quality) in an environment typically characterized by scarce resources, risk and uncertainty.
My focus in this post will be on time management. We all know we cannot manage time. Time actually manages us. But, what we can do is make best use of available time. CPM is the methodology of choice for this.
CPM was “invented” in the 1950s and has been used since that time across multiple industry areas and will continue to be used to plan and manage project activity for the foreseeable future.
A core concept in CPM is “Float”.
Float is the difference between the time established for completion of a project and the sum of task durations along the longest path (i.e. the Critical Path) from today/now to the project objective.
Float is a number expressed in hours, days or weeks. It can be positive, zero or negative.
Because CPM is deterministic, it is best in the planning of any project to establish a reserve, accordingly float should start off as a positive number. The ideal starting position is one where task durations and resource requirements have been estimated conservatively and where initial float is perhaps 20% of total project duration.
Because projects have long lead times from concept to the start of implementation, a common scenario is no initial float and tightly scheduled tasks at the time the project plan and schedule is published.
In any case, whatever your starting float is, the key to on-time completion of the project is to prevent erosion of that float.
When a project starts to run late, it is tempting to “improve” projected completion dates by compressing task durations. Changing the network logic by doubling-up tasks (e.g. build two prototypes and ship one to the customer before testing the second) is a second tactic often used. Both of these tactics decrease the usefulness of time projections.
Here are three practical tips for success with CPM:
1. Resist the temptation to compress forward time estimates to “improve” CPM calculations. Look first to the application of additional resources to improve time projections and to changes in network logic.
2. Where a project is running late along the Critical Path, look beyond Float along the critical path for an estimate of the amount of effort likely to be needed to “rein in” the project. (i.e. If there are four pathways consolidating to the project end node and the float readings are -10, -6, -4, -2, the project is less in jeopardy than a scenario with readings of -10, -8, -8, -8).
Saving four weeks in the case of the first scenario improves the float to -6 whereas saving two weeks along the critical path under the second scenario will put the project float at -8 weeks where it will stay until you save an additional 10 weeks to get the float to go to -6 weeks.
3. Try to find a CPM program that allows you to input planned start, planned finish, actual/projected start, actual/projected finish dates per task so that you can carry out data mining post-project with the objective of improving performance on the next similar project.
Stay tuned for a comparison between BPM and CPM. Specifically, when to use one, when to use the other, when to use both.