5 th LectureCEN 4021: Software Engineering II Task Sequence and Effort representation All tasks...

5th LectureCEN 4021: Software Engineering II

Task Sequence and Effort Task Sequence and Effort representationrepresentation

All tasks share at least two basic characteristics– Require effort in terms of person-days required to complete

each task

– A specified order for processing the task


Task sequence/effort tableTask sequence/effort table

Task Immediate prerequisite

tasks

Effort (person-days)

A None 9

B A 5

C A 7

D B,C 11

E D 8

Total effort required: 40 person-days


Graphical representation Graphical representation of tasks from previous of tasks from previous tabletable

Task A

Task B

Task C

Task D

Task E

End

Critical Task

Non-critical Task

Critical Path

Non-critical Path

9

9 5

7

11 8

Total project time: 35 days


Critical versus Non-Critical Critical versus Non-Critical PathsPaths

Critical path The path that takes the most time to complete.

Critical task (critical activity) A task that resides on the critical path.

Non-critical path Any path that is not a critical path and thus takes less effort to complete

than the critical path.

Non-critical task (non-critical activity) Any activity that resides on a non-critical path and not a critical path.

These tasks may accept some delay in completion.


Critical Path Method (CPM)Critical Path Method (CPM)“The critical path method analyses the precedence of

activities to predict the total project duration. The method calculates which sequence activities (path) has the least amount of flexibility ”– Quality Software Project Management, Futrell et. al.


Task SchedulingTask Scheduling Forward-pass scheduling

– Early start time (ES) and early finish (EF)

Backward-pass scheduling– Late start (LS) and Late finish (LF)

Slack time


Forward-pass schedulingForward-pass scheduling

Task Tasks precedence

Task length Earliest possible start time

(ES)

Earliest possible

finish time (EF)

A None 9 0

B A 5

C A 7

D B,C 11

E D 8

9

9 14

9 16

16 27

27 35


Backward-pass schedulingBackward-pass scheduling

Task Tasks precedence

Task length Late start time (ES)

Late finish time (EF)

A None 9

B A 5

C A 7

D B,C 11

E D 8

9

11 16

9 16

16 27

27 35

0


Slack timeSlack time

Total slack time of an activity– The difference in start time between a non-critical task’s late

start time and its early start time or its late finish time and early finish time.

Total slack time of a task = LS - ESOrTotal slack time of a task = LF – EFe.g., Activity B: LS – ES (11 - 9) or LF – EF (16 - 14) => 2

Total slack time– The maximum allowable delay for all non-critical activities.


Program Evaluation and Program Evaluation and Review Technique (PERT)Review Technique (PERT)

An estimating technique assumes each activity duration is subject to a range

estimates uses weighting method to arrive at a specific duration.



Expected estimate (EE) = [O + P + (4 x A)] / 6

WhereO = most optimistic estimateP = most pessimistic estimateA = most likely estimate



Expected Project time (EPT) Sum of expected estimates of the critical tasks.EPT = Σ(EE of all critical task)


CPM vs PERTCPM vs PERT Scheduling a lot of activities with 3 estimates

computationally messy many people argue 3 estimates are not much more

accurate than 1 estimate Most project management scheduling software use CPM

5 th LectureCEN 4021: Software Engineering II Task Sequence and Effort representation All tasks...

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