17spm_L10
Scheduling, Planning and Tracking
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PERT, Gantt and Earned Value Charts
Scheduling and Tracking
� Scheduling decides when to start a particular activity.
� Tracking is monitoring where you are and whether you are on
schedule.
� Several formal techniques can be used.
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o A PERT chart shows the sequence in which activities take place.
o A Gantt chart shows the start and end date of each activity.
o An Earned Value chart is a graph of elapsed time verses effort.
PERT and Gantt Charts
� PERT = Program Evaluation and Review Technique.
o Developed by USA Navy in 1950’s
o Also called critical path and network analysis.
� Gantt = technique invented by Henry Gantt.
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o American Mechanical Engineer and Management Consultant.
o Developed in 1910’s.
o Used for the Hoover Dam and Interstate Highway projects.
PERT Chart
� Shows the sequence of tasks and how they relate to other tasks.
� Each task may have some earlier tasks that it depends on and other
tasks that depend on it.
� Each task also has an elapsed time that it will take.
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� The chart is a graph of nodes connected by lines.
� The critical path is the shorted time that the project will take.
o All the nodes on the critical path must be done one after the other
with no time gaps between them.
Activities and Milestones
� An activity takes a certain amount of time.
� A milestone is the endpoint of an important activity and leads to the
start of the next activity.
o Must have a measurable outcome that is either true or false and can
be ticked off.
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be ticked off.
� A deliverable is delivered to a ‘customer’ who may be an internal
customer.
o Are usually milestones.
o Can be documents such as requirements spec.
o Can be software component or working system.
Resources
� People are the most important resource.
o Must be enough people with specialist skills.
� Too few specialist can make it difficult to:
o Carry out sufficient activities in parallel.
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o
o Cause a bottleneck for certain aspects such as testing.
� Training in the early part of a project may be an important activity.
o Deliverable is trained members of the team.
Project Planning
� Divide into separate activities.
o Define the activity.
o Identify milestone that will show that it has ended.
o Estimate the effort required.
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o
o Estimate the minimum elapsed time required.
� Determine the dependencies between activities.
o Which activities must be finished before the next can start.
� Find the critical path(s).
o Activities that must stick to their schedules.
� Find activities that have some slack time.
o Not on the critical path.
PERT
� Used to find the critical path(s).
� Find the minimum time for the whole project based on:
o Minimum times for each activities.
o Dependencies between activities.
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o
� Critical activities and those with some slack are revealed.
� Two forms of PERT chart. Both record the same information.
o Activity as a Node. I will use this style.
o Activity as an Arc
� The effort required for each activity is not needed to determine the
critical path. Just the elapsed time is important.
� It is useful for resource planning.
Example: PERT Chart Initial Information
Task Elapsed Dependencies
A 4
B 3
C 5
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D 1 C
E 4 B, D
F 5 A, E
G 5 B, D
H 6 C
Forward Pass
� Find the earliest time Ei that each activity can finish.
� Start from the beginning and follow all routes through the graph.
� The earliest time Ei for an activity to finish is the earliest time that the
previous activity could finish + the length of this activity.
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� If there is more than one route to get to the activity, take the maximum
value of the various Ei values.
� Record this in the activity.
Example: PERT Chart Forward Pass
Task Elapsed Dependencies Earliest Finish
A 4 4
B 3 3
C 5 5
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D 1 C 6
E 4 B, D 10 (max 7, 10)
F 5 A, E 15 (max 9, 15)
G 5 B, D 11 (max 8, 11)
H 6 C 11
Backward Pass
� Find the latest time Li that an activity can finish.
� Find the maximum finishing time for all activities.
� Start from the end and follow the graph backwards towards the start.
� The earliest finishing time for an activity is the earliest finishing time
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of the activity it leads to, minus the length of this activity.
� If more than one path leads from an activity, choose the smallest of the
Li values.
� Record this in the activity.
� A dependency graph helps at this stage.
Dependency Graph
A F
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E
G
H
DC
B
Example: PERT Chart Backward Pass
Task Elapsed Dependencies Earliest Finish Latest Finish
A 4 4 10 (from F)
B 3 3 6 (from E, G)
C 5 5 5 (from D)
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D 1 C 6 6 (from E, G)
E 4 B, D 10 (max 7, 10) 10 (from F)
F 5 A, E 15 (max 9, 15) 15
G 5 B, D 11 (max 8, 11) 15
H 6 C 11 15
Slack Time
� The slack time Si for each activity = Li – Ei.
� If the slack time Si = 0 then the activity is on the critical path.
� The critical path(s) are all paths connecting the critical activities.
� If any critical task is delayed then the project as a whole will be
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delayed.
� Free Float = amount of time an activity can be delayed without
affecting any of the other activities.
� Total float = amount of time an activity can be delayed without
delaying the project as a whole.
o Some other non critical activities may also be delayed.
� Slack time can also be used to spread an activity out longer than its
minimum time.
Example: PERT Chart Backward Pass
Task Elapsed Dependenci
es
Earliest Finish Latest
Finish
Slack Time
A 4 4 10 (from F) 6
B 3 3 6 (from E,
G)
3
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G)
C 5 5 5 (from D) 0
D 1 C 6 6 (from E,
G)
0
E 4 B, D 10 (max 7, 10) 10 (from F) 0
F 5 A, E 15 (max 9, 15) 15 0
G 5 B, D 11 (max 8, 11) 15 4
H 6 C 11 15 4
Dependency Graph: Critical Path
A F
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E
G
H
DC
B
Recording Information
� A node for an activity contains:
� Description:
� Duration:
� Earliest Start:
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� Latest Start:
� Earliest Finish: from earliest start and duration
� Latest Finish: from latest start and duration
� Float:
� Activity Span: actual duration, can be longer if non-critical.
PERT Chart for MSc SD
� Prog, DTA, SN, SPM, ECS: stand alone.
� AP, ADS depend on Prog.
� TeamProj depends on Prog, DTA, SPM.
� SE depends on SPM.
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� 2 options: stand alone.
� Exams depend on all above.
� Summer project depends on Prog and Exams.
� Elapsed time: 3 months for each activity.
� Effort: 10 hours per credit.
Gantt Chart
� The x-axis is time.
� Each different activity is listed on the y-axis
� Activities are long rectangles, with the critical path in black.
� Milestones appear as diamonds.
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� Activities not on the critical path can be shifted and lengthened, using
up the slack.
o They can’t be shortened.
� We can record the effort and staff required for each activity.
o Get an idea of employment over time.
Gantt Chart: MSc SD | IT
� Scheduling by semesters.
� Milestones:
o End December with option to switch to IT.
o End of exams with option to take resits, extend the program.
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o
o End of resits with option to take Diploma.
Earned Value Chart
� The x-axis is the elapsed time.
� The y-axis is the effort expended.
� The planned project work is one line on this graph.
� The actual progress is plotted as another line on the graph.
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� The difference between the two lines will show:
o Difference in effort expended
o Difference in elapsed time.
� Used to estimate actual completion time and cost.