Critical Path Analysis and PERT Charts An overview of the article by Simu Mihai Stelian An IV, CTI Full version http://www.mindtools.com/critpath.html
Critical Path Analysis and PERT are powerful tools thathelp you to schedule and manage complex projects.They were developed in the 1950s to control largedefense projects, and have been used routinely sincethen.These tools help monitor and improve scheduling forcomplex interdependent activities such as commonlyinvolved in developing software. During management ofa project, they allow you to monitor achievement ofproject goals. They help you to see where remedialaction needs to be taken to get a project back on course.
The benefit of using CPA within the planning process isto help you develop and test your plan to ensure that it isrobust. Critical Path Analysis formally identifies taskswhich must be completed on time for the whole project tobe completed on time. It also identifies which tasks can bedelayed if resource needs to be reallocated to catch up onmissed or overrunning tasks. The disadvantage of CPA, if you use it as thetechnique by which your project plans arecommunicated and managed against, is that therelation of tasks to time is not as immediately obviousas with Gantt Charts. This can make them more difficultto understand.
How to use the tool : As with Gantt Charts, the essential concept behindCritical Path Analysis is that you cannot start someactivities until others are finished. These activities needto be completed in a sequence, with each stage beingmore-or-less completed before the next stage can begin.These are sequential activities. Other activities are not dependent on completionof any other tasks. You can do these at any time beforeor after a particular stage is reached. These are non-dependent or parallel tasks.
Step 2 :Plot the activities as a circle and arrowdiagram In these, circles show events within the project, such asthe start and finish of tasks. The number shown in the lefthand half of the circle allows you to identify each oneeasily. Circles are sometimes known as nodes. An arrow running between two event circles shows theactivity needed to complete that task. A description of thetask is written underneath the arrow. The length of thetask is shown above it. By convention, all arrows run leftto right. Arrows are also sometimes called arcs.
Here the activities of Select Hardware and CoreModule Analysis cannot be started until High LevelAnalysis has been completed.
Each event circle also has a figure in the bottom,right hand quadrant. This shows the latest finish timethats permissible for the preceding activity if theproject is to be completed in the minimum timepossible. You can calculate this by starting at the lastevent and working backwards. The latest finish time ofthe preceding event and the earliest start time of thefollowing even will be the same for circles on thecritical path. The activities on the Critical Path must be veryclosely managed to ensure that activities arecompleted on time. If jobs on the critical path slip,immediate action should be taken to get the projectback on schedule. Otherwise completion of the wholeproject will slip.
Crash Action : You may find that you need to complete a projectearlier than your Critical Path Analysis says is possible. Inthis case you need to re-plan your project. You have a number of options and would need toassess the impact of each on the project’s cost, qualityand time required to complete it. For example, you couldincrease resource available for each project activity tobring down time spent on each but the impact of someof this would be insignificant and a more efficient way ofdoing this would be to look only at activities on thecritical path.
PERT (Program Evaluation and Review Technique) PERT is a variation on Critical Path Analysis that takes aslightly more skeptical view of time estimates made for eachproject stage. To use it, estimate the shortest possible timeeach activity will take, the most likely length of time, and thelongest time that might be taken if the activity takes longerthan expected. Use the formula below to calculate the time to use foreach project stage:shortest time + 4 x likely time + longest time----------------------------------------------------------- 6
Key Points : CPA is a powerful tool for assessingtasks, their parallelization, upper bound of shortesttime, resources and priority of activities within aproject. An effective Critical Path Analysis can make thedifference between success and failure on complexprojects. It can be very useful for assessing theimportance of problems faced during theimplementation of the plan. PERT is a variant of Critical Path Analysis thattakes a more skeptical view of the time needed tocomplete each project stage.