What are the rules for drawing networks?



The following rules are all logically based and should be thoroughly learned before attempting to draw networks.

1)      A complete network should have only one point of entry – a START event and only one point of exist – a FINISH event.

2)      Each activity is represented by one and only one arrow in the network. Every activity must have one preceding or ‘tail’ event and one succeeding or ‘head’ event. Note that many activities may use the same tail event and many may use the same head event

However no two activities can be identified by the same head and tail events. When two or  more parallel activities in a project have the same head and tail events, DUMMY ACTIVITIES are needed in constructing the network.

A dummy activity , which is used when two or more activities have same tail and head events is known as an IDENTITY DUMMY.

Dummy activities are also useful in establishing logic relationships in the arrow diagram that cannot otherwise be represented correctly. Such dummy activities are known as LOGIC DUMMIES. E.g. Suppose that in a certain project, jobs A and B must proceed C while job E is preceded by job B only.

(3)     No activity can start until its tail event is reached.

(4)     An event is not complete until all activities leading into it are complete.

This is an important rule an invariably has to be applied in examination questions.

(5)     ‘Loops’ i.e. a series of activities which lead back to the same event are not allowed            because the essence of networks is a progression of activities always moving onwards in time.

6)      All activities must be tied into the network i.e. they must contribute to the progression        or be discarded as irrelevant. Activities which do not link into the overall project are termed ‘danglers’.

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MT UVA- University, Vocational and Affiliated Education for BMS


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