Production Process

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