This critical path method was developed in 1957 and
it is suitable for both civil and mechanical engineering projects.
This technique is useful to determine how best to reduce the time required to perform production, maintenance and construction and also to minimize the direct and indirect cost of the project.
Using this critical path method C.P.M, a planning engineer will come to know the sequence of the various activities of the project.
Critical path network can be prepared using the below steps :
Preparation of network
Estimation of the expected time to perform each activity.
Determination of critical path schedule.
Interpretation of the results.
Important 12 technical terms used in critical path method ,C.P.M
The following are the 12 technical terms used in critical path method
Early start time (E.S.T)
Early finish time (E.F.T)
Late start time
Late finish time
Activity is part of the project. The network of the project is denoted by an arrow.
The start of the activity is indicated by the tail of the arrow and
the end of the activity is indicated by the head of the arrow.
For a given duration of an activity only one arrow is utilised.
The arrows on the activity are not represented to scale.
Along with the arrows, the duration of the activity is shown
Between 2 and 4 arrows activity C is shown ,and its 4 units of time duration is shown beneath the arrow.
The dummy activity is the activity which does not ustilise any time or any resources for the completion of activity .
An event is the stage or point at which all previous completed activities merge and
jobs coming out of this point needs to be finished.
At the junctions of arrows, they are represented by circles or nodes.
In sequential order, they are serially numbered.
The representation of the entire project activities in diagrammatically is termed as network.
On this diagram, The various projects jobs which are expected to be completed are shown in the order.
5. Early start time
An early start is the earliest possible time at which an activity may start.
6.Early finish time
Early finish time is the sum of the earliest start time of activity and the time required for its completion.
7.Late start time
Without delaying the project date, the most possible time at which an activity may start is termed as late start time.
8.Late finish time
The sum of the time required for completion and the late start time of activity is termed as late finish time.
9. Total float
The difference between the estimated duration and maximum allowed time for an activity is termed as total float.
Without disturbing the project schedule it is the span of time by which the activity can start.
It is expressed by letter S.
10. Free float
The span of time without disturbing the start of succeeding activities by which the activity completion can be delayed is termed as Free float.
It is expressed by S.F.
The events which have no float are termed as critical activities.
All the critical activities need to be finished on schedule.
Thejoining of the critical events in the network path is termed as critical path .
Basic rules for critical path method network:
The following are the main rules of Critical path method network.
The flow direction is indicated by arrows, which is normally accepted from left to right.
The length duration of the activity is not correspond to length of the arrow. It is just geometrical configuration and does not have any importance.
The activities merging at it must be completed before starting any new activity.
The indication of the logical conditions of dependency in a network is done by an arrow.
The construction of network is done logically on the principle of dependency.
For starting the project and also for closing the project only one event must be there in every network.
These connection of any two events may be done directly but not with more than one activity.
Serial numbers should be given to events and those numbers should not done duplication in a network.
Drawing a Network in critical path method
The following information is needed for drawing a network of any project.
List all the activities required for completion of a project.
Ascertain the dependence of the activities on each other.
Draw a network of the activities roughly with a soft pencil so that it can be easily erased.
The duration of each activity can be estimated by keeping in mind about the availability of equipment, manpower and also the likelihood of activities delays.
The estimate time of each activity needs to be noted.
For deciding the critical path,the latest and earliest allowable start and finish times for every activity to determined.
In the field need to finalise the network for use.
Advantages of critical path method
The important advantages of critical path method are:
It is useful to ascertain the time schedule
It helps to have proper control of the project.
It enables better and detailed planning.
It is very helpful to communicate the cost, project schedule, project planning ,performance ,time and project planning.
The critical activities which are highlighted along the critical path should be attended with care until their completion.
Despite the great planning cost of a project, Critical path proves to be very useful for focussing the attention of project engineers on the critical path activities and thus avoids unnecessary expenditure.
Expected length of the project , the critical path
1. Critical path :
The longest duration path of a network.
The sum of the expected times of the total activities is maximum in this path.
2. Length of the project
The sum of the expected times of all the activities along the critical path of the network of a project is called the length of the project.
3. The variance of the project
The variance of the project is the total sum of the variance along the critical path the activities of the network of a project.
4. The standard deviation of the project
The standard deviation is the square root of the variance which is calculated along the network critical path of a project.
Technical terms used in critical path method
1. Independent float
The free slack or independent float is the difference between the the minimum total float of all its immediate successors and the activities total float.
2. Total float
The total floats is the difference between the early and late finish times of an activity.
There will be zero total floats for the critical path activities.
An even or the mile stone is the point in time which represents the beginning of the completion of the same activity or group of activities on the project network.
The relationship between the trade-off and time-cost for an individual activity is assumed linear and expressed by a straight line on a graph plotting activity duration versus cost.
The straight line slope is called cost slope.
The greater is the cost of expediting the activity,the steeper the slope of this cot line.
As the duration reduced cost goes up which is indicated by the straight line which slopes down to right.
The line will be critical, If an activity time can’t be reduced even after extra resources applies to it.