Post on 17-Dec-2015
Managing time
‘Time is nature’s way of stopping everything happening at once’
Hughes & Cotterell 2002
Production Process
Lecture content
Recap on last week Scheduling What is Critical Path Analysis?
– Why use CPA?– Problems associated with CPA– How to perform CPA
Recap on last week
Stakeholders Identifying the goals of the project Defining tasks
– Activity based approach– Product based approach– Hybrid approach
Scheduling
Scheduling is the process of assigning tasks to a set of resources.
It is an important concept in many areas such as computing and production processes.
In mathematical terms, a scheduling problem is often solved as an optimisation problem, with the objective of maximising a measure of schedule quality.
Scheduling
Forward scheduling
planning the tasks from the start date.
Reverse scheduling
planning the tasks from the due date.
CPA & PERT
Critical Path Analysis and PERT are powerful tools that help you to schedule and manage complex projects.
Developed in the 1950s to control large defense projects.
Have been used routinely since then.
Why use CPA?
Shows the sequence of activities Shows dependencies Aids scheduling resources & financial
planning Used as a basis of control Diagrammatic therefore easier to read
What is Critical Path Analysis?
1. Using intertask dependencies, determine every possible path through the project.
2. For each path, sum the durations of all tasks in the path.
3. The path with the longest total duration is the critical path.
Problems associated with CPA
Pay too much attention to critical path Experience needed to gauge durations Difficulties in separating tasks Only deals with numbers Doesn’t guarantee good project
management, nothing does!!!
Start & Finish times
Activity = a task or action with a recognisable start/finish time e.g. ‘write report software’
Earliest start (ES) Earliest finish (EF) = ES + duration Latest finish (LF) = latest task can be completed without
affecting project end Latest start = LF – duration
Earliest startLatestfinish
Earliest finishLatest start
activity
Example
Earliest start = 5 days Latest finish = day 30 Duration = 10 days
Float = LF-ES-Duration
Earliest finish =? Latest start = ?
Float = ?
The total float of an activity is the amount of time by which the task may be extended or delayed without delaying completion of the project.
Notation
Activity description
Activity label duration
ES
LS
EF
LF
Activity span Float
Earliest start time/date
Earliest start date for the current activity = earliest finish date for the previous
When there is more than one previous activity, take the latest earliest finish
Note ‘day 7’ = end of work on day 7
Example
EF = day 7
EF = day10
ES = day 10
What is the ES for G?
A
B
C
D
F
E
G
ES = day 0
Duration 5
Duration 6
Duration 10
Duration 9
Duration 8
Duration 3
Duration 4
Complete the table
Activity ES duration EFABCDEFG
Latest start date
Start from the last activity Latest finish (LF) for last activity = earliest finish (EF) work backwards Latest finish for current activity = Latest start for the
following More than one following activity - take the earliest LS Latest start (LS) = LF for activity - duration
LS for all activities?
A
B
C
D
F
E
G
ES = day 0
Duration 5
Duration 6
Duration 10
Duration 9
Duration 8
Duration 3
Duration 4
Complete the table
Activity ES Dur EF LS LFABCDEFG
Now add the float!
Act-ivity
ES Dur EF LS LF Float
ABCDEFG
Answer
Find the Critical path
Note the path through network with zero floats
Critical path: any delay in an activity on this path will delay whole project
Can there be more than one critical path? Can there be no critical path? Sub-critical paths
Estimating task duration
1. minimum amount of time it would take to perform the task = optimistic duration (OD).
2. maximum amount of time = pessimistic duration (PD).
3. expected duration (ED) 4. Calculate the most likely duration (D) as follows:
D = (1 x OD) + (4 x ED) + (1 x PD) 6
Something to think about…
Can we truly say that by adding more human resource to a project it will reduce the time it takes to perform the task?
Further Reading
http://www.mindtools.com/pages/article/newPPM_04.htm
http://www.mis.coventry.ac.uk/~nhunt/cpa/listof.htm
http://www.waa-inc.com/projex/PERT/cpa.htm
http://www.gamedev.net/reference/business/features/criticalpath/default.asp