Original URL: http://www.theregister.co.uk/2010/11/19/pm_pert_cpm/
A race to the finish
Pert, critical path or crawl to the line?
Project management Would you run 145 miles from Birmingham to London non-stop for fun? Of course not, but most years about 80 “ultra” runners do just that. They share the loneliness of the long distance project manager.
These brave souls running the Grand Union Canal Race  risk exhaustion and collapse on some lonely stretch of canal but most of them complete the course in times of between 33 and 40 hours. Britain’s longest, and some say toughest, running race is a good analogy for most projects. Entrants discuss at length their game plans – what to eat and drink, how many stops to take, what support to call on, how to cope with stress, and so on. And of course they meet unforeseen circumstances, from getting lost to nipple rash.
The average project manager (PM) can probably relate to the problems of the typical Grand Union Canal Race competitor (if not the rashes). And like these runners, PMs try as hard as possible to build robust plans to reach the objective.
For the past 50 years or so, the question has been what is the best way to model any complex activity – and whether you need specific software to do it.
Though less favoured than in its heyday, Pert (program evaluation and review technique) and the critical path method (CPM) remain cornerstones in project modelling. Pert was invented by a couple of US Navy bigwigs in 1957 to manage nuclear submarine maintenance. Subsequently it was used on huge projects such as the Grenoble Winter Olympics. CPM, developed slightly earlier, is seen as the forerunner to Pert and is sometimes amalgamated within it.
Pert is best known for its network charts, derived from the raw data about all the activities to be undertaken. The chart acts as a metaphor for the flow of time (left to right) and provide an easy visual indication of the dependencies between activities.
Project plan: the stuff in red is the critical path.
Like our canal runners, the PM’s number one obsession when embarking on a project is estimating total duration. How fast can we go?
Pert’s primary deliverable, calculated automatically from the raw data, is the so-called critical path, or the minimum time needed to complete the project, and the listing of linked activities in that minimum path. Once the critical path is calculated, earliest and latest start and finish times for non-critical activities also fall out, and the implications for resourcing and scheduling.
Typically, you number activities or events sequentially (10, 20, 30...), allowing for later additions if needed. An event on the Pert chart is a block, linked to other events in the correct sequence, setting out the order in which events unfold.
The raw data consists of the activity name, and optimistic, pessimistic and expected durations. Different calculation methods may be used to estimate a duration time. Microsoft Project, for example, uses a weighted average from which the critical path is derived.
Once this is done “slack” (or “float”) becomes evident in those activities not on the critical path. Slack is the length of time an activity can be delayed without affecting the critical path.
The Gantt chart displays the critical path, colour-coded. The simplicity of the Gantt chart, and the clarity with which it shows the flow of time, makes it the preferred and primary chart on many projects. Change the data, and the critical path and the Gantt charge change too.
Let's look at it a different way
Same project, but zoomed into specific Activities.
But with the increasing power and flexibility of simpler software such as spreadsheets and specific PM applications, are Pert and CPM becoming less popular?
Bob Walker, Microsoft project technical sales specialist, believes they are. “Most of our customers don’t use it and are not trained to use it, even though it’s a good visualisation tool. Many prefer to create a work breakdown structure [WBS] using Visio, and then to generate a Gantt chart from that data. There are also misconceptions about critical path, with some believing it’s fixed. It’s not, of course,” he says.
WBS is a tool for planning the project as a series of levels arranged in a tree structure, often created using a spreadsheet. It uses a technique known as “progressive elaboration” to give increasingly granular details on the activities.
Dale Vile, managing director of FreeForm Dynamics, agrees with Walker - with reservations. “Projects nowadays are more inclusive, with more people involved, and dynamic. Keeping Pert models up to date is therefore often more trouble than it is worth,” he says.
“Critical path analysis, however, is still absolutely essential, whichever way you do it. Otherwise you cannot be confident of delivery times and cannot assess the impact of any potential slippage.
“Today many projects are planned on the basis of ‘just enough’ analysis, in other words to determine key dependencies – including critical path – but with no attempt to be exhaustive as Pert requires. The 80:20 approach to planning is a lot more common.”
Grand Union Canal Race runners know this method of project management. Jason French finished seventh in 2007 in a gut-busting 34 hours: “I have a pre-race plan but due to an inability to discipline myself the plan is always: run until I can’t run any further, then stagger until I can’t stagger any further, then crawl until I can’t crawl any further. Hopefully, somewhere before I reach the end of that plan I will have crossed the finish line in a decent position.” ®