03 November 2020 Lean Production: Critical Path Analysis 3. 15 Lean Production Objectives; • Understand the key requirements of critical path analysis (CPA) • Understand the purpose, benefits and limitations of using such a tool • Apply the principles of CPA to a scenario
An age old problem… How do you make the perfect cup of tea? o Milk then water? o Sugar before milk? What’s your theory?
Critical Path Analysis – A decision making tool “…a method of organising the activities in a project to find the most efficient method of completing the project without wasting resources (time, money, materials)” Aims of CPA… n n n To plan complex operations To identify when a project must commence To illustrate the order of activities To identify critical activities – which are those activities which must be completed first To identify the times when resources can be reallocated
Nodes… they even grow on trees! The node shows the physical start and finish of an activity or task 1 Nodes are NUMBERED from left to right 1 3 2 5 4 6
Arrows and Nodes A 1 2 3 mins Activities are denoted by an arrow between nodes The duration of the activity is shown below the arrow, and could be minutes, days or another measure Node 1 shows the beginning of Task 1, Node 2 shows the end of Task 1 Other tasks can be started from Node 2 Activities are labeled above the arrow by a LETTER
Linking tasks A 1 3 3 mins D 1 min 5 B in 1 m m in 1 C 2 mins 4 E 2 Some tasks can be undertaken simultaneously, whilst others have ‘prerequisites’ which must be completed first e. g. making a cup of Mint Tea… A – Put the kettle on B – Get a suitable cup C – Place bag in Cup D – Pour Hot water and ‘infuse’ E – Remove teabag…enjoy!
This te are cap lls us t a h the ble of f at we i job afte nishing r 6 m Earliest Start Time A 3 3 mins D 3 2 mins B in m in 2 1 1 m 1 C 1 0 The top right box shows the earliest time than an activity can start – the earliest start time (EST). All previous tasks must be completed first 4 5 E 1 min 5 6 • Node 1 is always ‘ 0’. It can be started straight away! • Node 2 is only reached after 1 minute (task ‘B’) – so EST is 1 minute • Node 3 is only reached after 3 minutes (tasks B+C = 2 but task A = 3) • Node 4 is reached after 5 minutes • Node 5 is reached after 6 minutes
For L star FT we A t fro m th LWAYS e rig ht! Latest Finish Time A 0 3 3 mins 3 3 B in m 2 1 1 m in 1 C 1 0 2 The lower right box shows the latest finish time (LFT). This shows when we need to start the next task so as not to delay the project D 2 mins 4 5 5 E 1 min 5 6 6 • Node 5 here is always the same as EST, as it can’t be reached before and if it is later then the whole project is delayed! So ‘ 6’ • Task E takes 1 minute, so to be ready for LFT of ‘ 6’ at Node 5, its LFT is ‘ 5’ • Task D takes 2 minutes, so to be ready for LFT of ‘ 5’ at Node 4, its LFT is ‘ 3’ • Task C takes 1 minute, so to be ready for LFT of ‘ 3’ at Node 3, its LFT is ‘ 2’ • Task B takes 1 minute, so to be ready for LFT of ‘ 2’ at Node 3, its LFT is ‘ 1’ NOT QUITE! There are two tasks to consider here… • Task A takes 3 minutes, so to be ready for LFT of ‘ 3’ at Node 3, its LFT is ‘ 0’ Where LFT is calculated using more than one activity – we always allow for the longest route. In addition Node 1 always has an LFT of ‘ 0’
The float o With the network complete, you can now identify any idle time. o This spare time is known as the float. o Resources can be allocated to other duties during the float time. o How do you calculate ‘float’? LFT – (duration + EST)
Critical Path Analysis The NUMBER Board 1 5 2 6 3 7 4 8
1 What does an arrow represent in CPA? Number Board
2 What is this called, and when is it used? Number Board
3 What information is displayed above and below the ‘arrow’ in CPA? ? Number Board ?
4 What goes in here and what does it show? Number Board
5 Letter and numbers – each represents something specific in CPA, but what is it? Number Board
6 What is the name given to the series of tasks / activities that if delayed will delay the whole project? Number Board
7 Twigs float, so does polystyrene. But what does ‘float’ mean in a Critical Path Diagram? Number Board
8 What goes in here and what does it show? Number Board
Critical Path Analysis CPA is a planning tool – not a decision making tool o Advantages n n n Maximise efficiency in the use of time Improve efficiency and generate cost saving in the use of resources Beneficial to monitoring cash flow Identify potential problems in implementing operation Identifies where and when resources (including human ones) are needed o Disadvantages n n n Usefulness may be limited in complex and large scale operations Necessity of having clear and reliable information Skilled management and team philosophy is essential