- PERT was developed in 1956–58 by a research team to help in the planning and scheduling of the US Navy’s Polaris Nuclear Submarine Missile project involving thousands of activities.
- This technique has proved to be useful for projects that have an element of uncertainty in the estimation of activity duration.
CPM (Critical Path Method)
- CPM was developed by E.I. DuPont company along with Remington Rand Corporation almost at the same time, 1956-58.
- This technique has proved to be useful for developing time-cost trade-off for projects that involve activities of repetitive nature.
- In Project Planning phase, identify various activities (tasks or work packages/elements) to be performed in the project, that is, develop a breakdown structure.
S.No. | PERT | CPM |
---|---|---|
1. | PERT is that technique of project management which is used to manage uncertain (i.e., time is not known) activities of any project. | CPM is that technique of project management which is used to manage only certain (i.e., time is known) activities of any project. |
2. | It is event-oriented technique which means that network is constructed on the basis of event. | It is activity-oriented technique which means that network is constructed on the basis of activities. |
3. | It is a probabilistic model. | It is a deterministic model. |
4. | It majorly focuses on time as meeting time target or estimation of percent completion is more important. | It majorly focuses on Time-cost trade off as minimizing cost is more important. |
5. | It has non-repetitive nature of job. | It has repetitive nature of job. |
- Activities in the network diagram represent project operations (or tasks) to be conducted.
- Each activity except dummy activity requires resources and takes a certain amount of time for completion.
- An arrow is commonly used to represent an activity.
- The activities can be further classified into the following three categories:
- Predecessor Activity: An activity which must be completed before one or more other activities start is known as predecessor activity.
- Successor Activity: An activity which starts immediately after one or more of other activities are completed is known as successor activity.
- Dummy Activity: An activity which does not consume either any resource and/or time is known as dummy activity.
Fulkerson’s Rule – Numbering of events
- The initial event which all outgoing arrows with no incoming arrow is numbered ‘a’.
- Delete all the arrows coming out from node ‘1’.
- Delete all the arrows going out from these numbered events to create more initial events.
- Continue until the final or terminal node which has all arrows coming in, with no arrow going out is numbered.
Forward Pass Method (For Earliest Start Time)
- Set the earliest occurrence time of initial event 1 to zero. That is, E1 = 0, for i = 1.
- Calculate the earliest start time for each activity that begins at event i (= 1). This is equal to the earliest occurrence time of event, i (tail event). That is:
ESij = Ei , for all activities (i, j) starting at event i.
3. Calculate the earliest finish time of each activity that begins at event i. This is equal to the earliest start time of the activity plus the duration of the activity. That is:
EFij = ESij + tij = Ei + tij , for all activities (i, j) beginning at event i.
4. Proceed to the next event, say j; j > i.
5. Calculate the earliest occurrence time for the event j. This is the maximum of the earliest finish times of all activities ending into that event, that is,
Ej = Max {EFij} = Max {Ei + tij }, for all immediate predecessor activities.
6. If j = N (final event), then earliest finish time for the project, that is, the earliest occurrence time EN for the final event is given by
EN = Max { EFij} = Max {EN – 1 + tij}, for all terminal activities
Backward Pass Method (For Latest Finish Time)
Float (Slack) of an Activity and Event
(i) Optimistic time (to or a) The shortest possible time (duration) in which an activity can be performed assuming that everything goes well.(ii) Pessimistic time (tp or b ) The longest possible time required to perform an activity under extremely bad conditions.(iii) Most likely time ( tm or m ) The time that would occur most often to complete an activity, if the activity was repeated under exactly the same conditions many times. Obviously, it is the completion time that would occur most frequently (i.e., model value).
- Total float
- Free float
- Independent float
- Interfering float
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