3.4 Planning
3.4.3 Critical Path Method (CPM)
Both the Critical Path Method (CPM) and Program Evaluation and Review
Technique (PERT) are developed from simple network diagrams. The network consists of lines which represent activities and nodes (round circles) that
represent the beginning or end of an activity, sort of like the Paris Métro system maps.
A"Simple"Network"Diagram"
Figure 10 Network Diagram
Simple network diagrams are the basis for CPM and PERT. While both systems were developed separately in the 1950s in the United States -‐ PERT, by the U.S.
Navy and consultants Booz Allen to manage the Polaris missile project and CPM by employees of DuPont and Remington Rand (Wikipedia, n.d.) -‐ there is enough similarity to consider them of the same family of critical path users. (Fontana, 1990)
Critical Path Analysis refers to analyzing the network to find the shortest time for project realization. (Fontana, 1990) The critical path is the sequence of stages determining the minimum time needed to complete a project by plotting out the longest timeline. If the line is too long, then each activity is revisited to see how and where time requirements can be improved. Using CPM, DuPont was able to reduce from four years to two-‐and-‐a-‐half years the time required to close a chemical plant for maintenance and reopen for production.
Nowadays, CPM is used for repetitive project planning, when reliable estimates of time and/or the deadline are known. PERT is preferred if no reliable time estimates are available, but a cost analysis is essential to reduce project time.
Both CPM and PERT have two phases:
1) qualitative, involving an inventory of tasks required to attain the goal, establishing dependency of tasks, and designing the network; and
2) quantitative, establishing the time required to finish each task by specifying both the earliest and the latest possible termination dates.
It is usual for students to want to rush to the second phase, to spontaneously draw the arrows and nodes of a network. But this will cause problems and errors in planning, so it is important to spend sufficient time brainstorming the inventory before attempting to apply chronology.
At ESH, we use a Creative Problem Solving ♥-‐derived method of "breathing"
(Wolfe, 2007) to brainstorm in order to sufficiently develop the qualitative task inventory in phase one.
Figure 11 "Breathing" creativity (Wolfe, 2007); drawing by C. Konold
Thoroughness rather than chronology takes precedence in this step. Then we look at dependency of tasks which will thus establish the order in which tasks may be undertaken but allowing for parallel activity.
The easiest way to do this before putting anything to paper is to jot the tasks' key words on large-‐sized Post-‐Its ("papillons") which can then be easily plotted on the classroom wall. This juggling process is important to elicit discussion on the team of how and when things will be done. More actions may be discovered and added to the inventory. (For complex projects, a ball of twine may be used to express the inter-‐relatedness of each task.) This is the planning rough sketch.
When your rough draft has been sufficiently developed, you can complete a project task inventory as follows, where tasks are each given a letter of the alphabet and dependency is entered into the right-‐hand column.
Sample'CPM'or'PERT'Task'Inventory'on'Home'Moving
So,$you$decide$to$move…
Code Task'/'Activity Dependency
A Get$bank$loan$to$pay$deposit,$movers 0
B Look$for$a$new$apartment 0
C Sign$new$lease A
D Get$estimates$from$movers B,$A
E Give$notice$to$landlord$that$you$are$moving C
F Pack$belongings$into$boxes D
G Cancel$or$transfer$contracts$(EDF,$phone…) E
H Move F
I Pay$movers G,H
J Settle$into$new$premises H,$I
N.B. Do not attemtp to establish chronology while taking this inventory.
Only the resultant task dependency will determine chronology. When the Task list is complete, take a look at each task to determine what it is dependent upon. It may be dependent on several other tasks. Mark those letters in the dependency column. Do not attempt to draft the network until you$have$established$the$dependencies.
Figure 12 Sample Task Inventory with dependencies)
Using the task inventory, plot out the tasks on paper from left to right. Arrows ( ) represent tasks and circles ( ) or boxes ( ! ) denote "nodes" (beginning and end of tasks). Each arrow is labeled with its activity code (letter) written above the line. Time Estimates (TE) for completion of each task are written below the task arrow. A relationship between nodes is indicated by a "dummy"
line or dotted arrow (-‐-‐-‐ ) to show that a particular action must be finished before the one that the dummy line points to can be completed.
!!
Get!es&mates!
from!movers! Moving'House'Network'
!!!
!!!!
Get!ban k!loan!
Pack!! Find!ne
w!!
apartment! !!!
!!!
Sign!new!
lease!
!!
Give!
no&ce!
!!!
4! !!
Move!
1!
Key:!
Arrow!=!ac&vity!
Circle!or!“node”!=!event!(end!or!beginning)!
TE!or!Time!Es&mate!–!in!days!(red)!
N.B.!There!is!no!propor&onality!intended!in!the!length!of!lines!
10!
15!
2! 1!
7! !5!
2!
Change!
u&lity!
contracts!
Pay!!!
SeSle!in! !!!
1!
Figure 13 Sample CPM "Moving House"