Fundamental concepts used in CPM network

·         Some of the fundamental concepts used in CPM network are as follows:

o   Network – it is a diagrammatic representation of a project’s different activities and their logical interrelationships between the various activities. Arrows and circles or squares are used in the diagram. They represent activities and events respectively.

o   Activity – activity is task or activity to be performed. It takes time for completion. Activities are represented by arrows which are usually drawn from left to right. The length and shape of the arrow have no meaning. For example: excavate foundation, lay brick work, backfill trench, fix shuttering etc. For example:

o   Event -  it represents a circle (node). Event represents starts or completion of a task. It consumes no time or resources. Events or nodes of a network are numbered. For example: excavation completed, brick work laid, shuttering fixed, etc. There are two types of events:

§  Tail event – it is the beginning of an activity.

§  Head event – it is the completion of an activity.

§  Dual role – event is the event when it becomes tail event to some activities and head event to other activities. Consequently, all events except initial and final event of a net work diagramme will be dual role events.

o   Critical path – it is the path in a network diagram which takes maximum time (i.e. the longest duration) among the alternative paths through which a project can be completed and the time along this path determines the completion time of the project.

o   Critical activities – these are the activities lying on the critical path. Delay, if occurred, in a critical activity causes delay in the completion of the entire project. Activities which have zero total float are called critical activities.

o   Non – critical activities – activities having some float are not critical since the available time is in excess of the required activity duration. These activities are termed as non – critical activities. Delay in a non – activity up to the available total float will not delay the project completion. Resources may, therefore be optionally utilized by diverting some resources from non- critical to critical activities.

o   Float – it indicates the spare time available for the activities for their completion without delaying the completion of a project. In other words, it is the difference between the time available and the time required to perform the jobs.

o   Total float – it is the difference between the latest finish time or earliest start time respectively.

o   Free float – it is an obliged float within which an activity can float move without affecting the succeeding activities.

o   Independent float – it is the excess of minimum available time over the activity duration.

o   Dummy – it shows a dependence among activities. It occurs when two parallel activities have the same tail and head event and in the case of two activities independent to each other but assumes a common event. It is also called a Redundant Activity is represented by a broken arrow.

o   Earliest Event time ( T_E ) – it is the earliest possible occurrence of an event, i.e. earliest possible time when all activities leading to an event will be completed..

o   Latest event time (T_L ) – it is defined as the latest possible occurrence of an event without delaying the project completion time, i.e. is the latest time when all activities leading to an event may be completed without delaying the project completion time.

o   Earliest start time (EST) – it is the earliest time when an activity can commence, since the starting point of an activity is the “tail event”, the earliest start time of an activity is the Te of the “ tail event”.

o   Earliest finish time of an activity (EFT) -  it is the earliest time when an activity can be finished.

o   Latest finish time – it is the latest time when an activity may be finished without delaying the completion time of the project. Since the finishing point of the activity is the “ head event”, the latest finish time of the activity is the same as T_L of the head event.

o   Forward pass – for calculating the earliest event times (T_E), computations are made in a forward direction (left to right). This process is called the “forward pass”.

o   Backward pass – for calculating the latest event times (T_L), computations are made in a backward direction (right to left). This process is called the “backward pass”.

o   Slack time – slack time can be defined as the difference between the latest allowable date and the earliest expected date. For example:

§  T_E  =  the earliest time (date) on which an event can be expected to take place.

§  T_L = the latest date on which an event can take place without extending the     completion date of the project.

§  Slack time  =  T_L  -  T_E