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1

What is the concept of the critical path method?

The Critical Path Method (CPM) is the scheduling method used in virtually all project management computer software today. It was developed by DuPont and Remington Rand in the 1950s to improve project scheduling techniques. It is a modelling process that defines all the project’s critical activities – those activities that must be completed on time if the entire project is to be completed on time and calculates the start and finish dates of all activities in the project. With CPM, calculations of the schedule are based on the duration of the activities and the relationships between the activities. The schedule is calculated twice - first from the earliest start date forward, and then from the latest finish date backwards. The differences between the pairs of start and finish dates for each activity is float or slack time for the activity. Float (or more precisely Total Float) is the amount of time that an activity can be delayed without delaying the project completion date. The critical path is that sequence of activities which represent the longest total time required to complete the project. A delay in any activity on the critical path causes a delay in the completion of the project. Critical activities - activities on the critical path - have no slack time or Zero Float. Non-critical activities - activities not on the critical path - have a slack time or Positive Float

2

What are the steps in the critical path analysis?

>prepare a list of activities
>draw network of logical relationships
>estimate duration of activities
>determine earliest start and finish times by means of forward- and backward-passes
>determine the critical path and the activity floats

3

How is the activity duration estimated?

>Determine the quantity of each item; by doing a Material Take Off (MTO) from the drawings and/or from the Bills of Quantities (BoQs),
>Determine the resources required; usually from experience of managing these types of resources, and
Determine the unit output of each resource. This is by far the most difficult but may be accomplished: i. from experience or by using historical production records (sometimes documented as an estimating manual); ii. by using published “standard” outputs; iii. from work study, although this only is used with repetitive processes

4

What are the four types of dependency for time lags?

>Finish-to-start the start of an activity depends on the finish of its predecessor or
preceding activity(ies). This is the normal dependency.
>Finish-to-finish the finish of an activity depends on the finish of its predecessor or
preceding activity(ies).
>Start-to-start the start of an activity depends on the start of its predecessor or
preceding activity(ies)
>Start-to-finish the finish to an activity depends on the start of its predecessor or
preceding activity(ies)

5

What is the earliest start time and the earliest finish time?

Earliest start time of an activity (ES) is the earliest possible time at which an activity can start without affecting either the total project time or the logic of the network. So the earliest start time of an activity is that which allows ALL the ACTIVITIES which must immediately PRECEDE that activity, to be finished.
Earliest finish time of an activity (EF) is the earliest possible time at which an activity can finish without affecting the total project time or the logic of the network. The earliest finish time for an activity is therefore its earliest start time ES, plus its duration.

6

What is the forward pass?

The earliest start time of each activity is written at the left side of each activity box, and the earliest finish time in the right hand side for each activity node. This is based on an earliest start time of the first activity of zero.

7

What is the backward pass?

This is based on the fact that the latest finish time of the final activity is equal to its earliest finish time, i.e. the overall project duration. So the latest finish time of an activity is that which allows ALL the ACTIVITIES which must immediately succeed that activity, to be finished without delay to the project completion.

8

What is the critical path?

Clearly the overall project duration is established by determining the longest path through the network with the forward pass. This path is called the CRITICAL PATH since an increase in the duration or any activity lying on it will increase the overall project duration.

9

What are critical activities?

Activities and events lying on the critical path are called CRITICAL ACTIVITIES and any delay in the completion of a critical activity (e.g. an increase in duration) or in the achievement of a critical event will automatically increase the duration of the project by an amount equal to the increase in the duration or the activity or delay in achievement or the event. In a network diagram, the critical path contains those activities with zero total float.

10

What is a float?

NON-CRITICAL activities are said to have FLOAT, which is the term for time or schedule contingency. The various types of float which have been defined are useful for developing resource scheduling rules.

11

What is the total float?

Total float is the greatest time beyond its earliest start time by which an activity can be delayed, or the amount by which its duration can be increased, without causing a delay in subsequent activities. It is therefore the difference between the LS and the ES and is indicated at the bottom of the activity box.