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? • Using intertask dependencies, determine every possible path through the project. • For each path, sum the durations of all tasks in the path. • 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!!!
Latest finish Earliest start activity Earliest finish Latest start 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 start = 5 days Latest finish = day 30 Duration = 10 days Float = LF-ES-Duration Earliest finish =? Latest start = ? Float = ? Example 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 EF LS LF Float Activity span
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 Earliest start time/date
Example EF = day 7 ES = day 10 EF = day10
What is the ES for G? B E Duration 6 Duration 8 G A C F Duration 4 Duration 5 Duration 10 D Duration 3 ES = day 0 Duration 9
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? B E Duration 6 Duration 8 G A C F Duration 10 Duration 4 Duration 5 D Duration 3 ES = day 0 Duration 9
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 • minimum amount of time it would take to perform the task = optimistic duration (OD). • maximum amount of time = pessimistic duration (PD). • expected duration (ED) • 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