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activity $ begin.

A Simple Example#n many cases an activity will be dependent on the completion of more than

one preceding activity. #n the example shown the two activities A and * mustboth be completed before activity $ can begin. A requires % days * requires

& days and $ requires & days.

Activities A and * can be carried out in parallel as they are not dependent

upon each other. #n this example this part of the pro!ect will also last ' days.

+etermining the relationships between activities can be a complicatedprocess and may require a substantial amount of discussion involving

numerous personnel across the various departments that may be concerned.

The process of identifying relationships between the activities should only beconcerned with logical requirements in other words it should be assumed

that there are no resource constraints when drawing the PERT chart. This

issue has then to be addressed and appropriate ad!ustments made. Resolvingresource shortfalls and conflicts is the scope of resource planning and

scheduling.

+iagrammers may find it useful to produce sub,diagrams and use these to

conduct a brainstorming approach to identifying all possible relationshipsprior to building the final network. The pro!ect will need to be monitored at

various points to ensure that its business and technical integrity is beingmaintained , the PERT chart should also reflect these activities.

PERT charts contain detailed information relating to the activities necessary

to produce the required products. #t should be developed with reference tothe product flow diagram.

The top line of the activity box reflects the earliest time at which the activity

could start and finish. The centre line should contain descriptive information

about the activity and the bottom line should be used to reflect the latest



start and finish times. The lines that !oin activity boxes are used to representdependency between the activities. Identifying Spare TimeReading through the PERT chart from left to right gives the total duration of

the plan. Reading back through the network subtracting each duration

shows those activities that have any spare time. This spare time known asfloat is a very useful concept in relation to resource scheduling andsmoothing.

#n the example shown activity R is dependent upon the completion of both ofthe activities P and -. owever activity P is scheduled to take % days whilst -

should be completed in / days.

The area of work represented by this part of the network should last 0/ days.Activity - has a float of 1 days , so activity - could start up to 1 days late or

take an extra 1 days to complete without delaying the start of the

subsequent activity 2R3.

Reading back through the PERT chart subtracting each duration showsthose activities that have any spare time. This spare time known as float

identifies parcels of time that can be used to introduce flexibility into theproduction sequence.

All planning should be iterative and the drawing of the PERT chart mayhighlight further products which need to be added to the work breakdown

structure and the product flow diagram.

The Critical Path)nce the resources required to complete each product have been identified

and the PERT chart updated to reflect these then the start and finish datescan be added. 4ith the shape and si(e of the pro!ect now visible the total

cost of the resources for each planning period can be calculated.



The PERT chart also clearly identifies the critical path 2shown in red above3which is the sequence of related activities which will take the longest time.

The critical path is an invaluable concept in pro!ect planning , as it definesthat sequence of activities that should take the longest time. Total & Free Float

The above network represents a small pro!ect. The activities that are on thecritical path 2shown in red3 have no float whilst those that lie off the criticalpath can have an associated float.

#n addition to the float associated with a specific activity two other types offloat need to be considered.



Total 5loat 2shown in yellow above3 is shared with all of the other activities inthe arm. Therefore using the float in one activity will reduce the float

available for the other activities. 5or example in the arm P- the total float is0'. This is because the dependent activity 6 has a latest start of /1 and the

total duration of the P- arm is &. That is /1 minus & equals 0'.

5ree 5loat 2shown in yellow above3 is the amount of float that the activity canuse up without affecting the earliest start of any other activity. )nly activities

that precede a !unction can have free float. 5or example the earliest finish of

activity - is &. The earliest start of the dependent activity 6 is 0/. Thereforethe completion of activity - could be delayed for a maximum of 7 without

affecting the earliest start of activity 6.

5loat is a measure of the amount of time an activity can be delayed withoutaffecting subsequent activities. The amount of float indicates the extent of

time that the activity can be delayed without putting back the end date of theoverall pro!ect. Activities that have an associated float are natural candidates

to be delayed when other activities are suffering from problems or overruns.

The critical path 2shown in red3 is defined as the series of activities which

have (ero float. There will always be a critical path running through a pro!ectfrom the first activity to the last. owever any task if sub!ected to sufficient

delay may itself become critical , and this will occur at the point when its



float has been entirely consumed by the passage of time.

#n larger pro!ects particularly as the pro!ect nears completion a number ofthe arms of the network may contain (ero float i.e. the pro!ect may contain

numerous critical paths. The critical path is an important feature in pro!ectplanning and control and is usually highlighted on the network in some

manner , e.g. bold print red or a different style of line. #t is important toremember that the critical path is not defined at the initial planning stageand then set in stone.

As the pro!ect progresses and planned activities overrun the network shouldbe frequently updated to ensure that it continues to reflect the true status of

the pro!ect. The failure to do this is a common reason for pro!ects going outof control and ultimately failing.

Pert Charts Free !nline T"torial

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