5+-+Critical+Path+Analysis

=Critical Path Analysis= Use the following sections to help you understand the different aspects of critical path analysis.

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=Drawing Network Diagrams= Key points: media type="file" key="d14actnet.swf" width="642" height="642" media type="youtube" key="mLI-UR5tXlI" height="450" width="600" media type="youtube" key="61wQcqgG0Sc" height="450" width="600"
 * For this topic the activities are represented by arcs and so diagrams are different from the graphs you have drawn in other topics;
 * Activites are on arcs, events are on the nodes;
 * You cannot have more than 1 activity starting and finishing at the same node - this is why we use a dummy activity with weight 0;
 * You must be able to explain why a dummy activity is needed - check the mark schemes carefully as they are usually 2 marks!

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=Critical Path Analysis= You need to be able to carry out a forward pass and backward pass on an activity network using early and late event times.

__**Early event time**__ = The earliest that you can move on from an event given that all the preceding activities have been completed. Complete this on the forward pass for each event by considering the earliest time that you can move on from that event making sure ALL activities leading up to it are completed.

__**Late event time**__ = The latest time that an event can be completed without extending the time needed to complete the project. Complete this on the backward pass, considering the latest time that a particular event can take place and still be completed in time. media type="youtube" key="a4eu05QRetg" height="360" width="640"

__**Critical activities**__ are activities that if overdue will make the whole project overdue.

If there is a path from start to finish of critical activities then this is called a __**critical path**__.

For each event on the critical path: early event time = late event time.

When there is a difference between the early and late times you have some breathing space before any delays affect the whole project. See float time below. media type="file" key="d14cpa.swf" width="645" height="645"

media type="file" key="d14ca.swf" width="650" height="650"

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=Float Times=

The total float of an activity is the amount of time that it may be delayed without affecting the overall project.


 * __Total float__ = __latest finish time__ - __duration__ - __earliest start time__**

You need to look at the early start time at the event BEFORE the activity and the late finish time at the event AFTER the activity for this calculation.

Example: Look at this network where the double box represent early start times and late start times for each event. If we zoom in and calculate the float times for particular activity arcs: The total float of A = 3 - 0 - 0 = 0. This is because A is a critical activity. All critical activities have a float of 0 as there is no give in their completion time. Total float of C = 11 - 4 - 3 = 4. 3 is the earliest C can start. 11 is the latest C can finish. C will take 4 to complete. This means their is a give or float of 4. C can be delayed up to 4 time units and still not impact on the overall finish time of the project.

Total float of F = 16 - 4 - 7 = 5. 7 is the earliest F can start. 16 is the latest it can finish, but it only takes 4 to complete. This means that F can be delayed up 5 time units and not impact on the rest of the project. media type="file" key="d14totalfloat.swf" width="653" height="653"