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Guide to the Critical Path and Critical Path Analysis
 

Guide to the Critical Path and Critical Path Analysis

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Instructions to manually calculate the Critical Path of a project.

Instructions to manually calculate the Critical Path of a project.

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  • Change sequence and add random durations. Then recopy and repaste all drawings and revise calculations so there is an obvious critical path.

Guide to the Critical Path and Critical Path Analysis Guide to the Critical Path and Critical Path Analysis Presentation Transcript

  • Schedule Build Review Critical Path Analysis ©Focus Planning Ltd
  • Disclaimer Information contained within this presentation is for education purposes only. How a programme or schedule is built, maintained and managed is the responsibility of the owning organisation. Focus Planning Ltd accepted no responsibility for changes made to programmes or schedules which are altered as a result of reading slides contained within this presentation. The configuration and settings of computer software are the responsibility of the license holders and Focus Planning Ltd accept no liability for the configuration used by the license holder.
  • What is the Critical Path? The Critical Path is a sequence of logically linked activities for which the sum of these activities dictates the overall duration of the project. The Critical Path in most project management software’s is also deemed as being the one that has zero float across the sequence of activities. The below example shows a sequence of activities, for which the activities with a red border represent the Critical Path, as that sequence in the schedule meets the above criteria; 3© Focus Planning Ltd
  • What is Critical Path Analysis? Critical Path Analysis is the procedure used to calculate the critical path in a schedule or network diagram, and also to calculate the float. Most, if not all Project Management and Planning software will calculate this automatically for the user, but it is important to understand how this is done so the plan can be clearly communicated and reviewed by all stakeholders. In order to calculate the Critical Path two calculations take place; 1. The Forward Pass: Calculates the Early Start (ES) and Late Start (LS) dates for activities 2. The Backward Pass: Calculates the Early Finish (EF) and Late Finish (LF) dates for activities These dates can be defined as; Early Start = The earliest an activity can start given the logic and constraints of the path Late Start = The latest an activity can start given the logic and constraints without delaying the project Early Finish = The earliest an activity can finish given the logic and constraints of the path Late Finish = The latest an activity can finish given the logic and constraints without delaying the project 4© Focus Planning Ltd
  • Preparing a manual analysis Activity Node Layout Linking Nodes ©Focus Planning Ltd 5 The simplest way to calculate the Critical Path is to display the activities as nodes, in the logical order to which they need to be delivered.
  • Preparing a manual analysis ©Focus Planning Ltd 6 Below is the earlier example taken from the Gantt Chart and converted in to Nodes;
  • The Forward Pass: First Activity The first activity in this example has no predecessor as it is the start of works on site, so the Early Start Date is day zero. To calculate the Early Finish, take the Early Start Date and add the duration, in this case; 0 (ES) + 5 (D) = 5 (EF) Enter the values in to the Node and continue on to the second activity. 7© Focus Planning Ltd
  • The Forward Pass: Second Activity Activity 2 has only one predecessor on a Finish-To-Start relationship, that being Activity 1 must finish for Activity 2 to start. Because of this relationship Activity 2 has an early start date equal to the Early Finish date on Activity 1, and as Activity 2 takes 2 days the early finish is the same calculation used before; Early Start + Duration = Early Finish. In this case 5 (ES) + 2 (D) = 7 (EF). Continue to calculate the Early Start and Early Finish for all the remaining activities in the schedule, remembering that in this example with all Finish-To-Start logic that Early Start = Predecessor Early Finish, and that Early Finish = Early Start + Activity Duration. If there is more than one predecessor then use the latest Early Finish of the predecessors as the Early Start for that node. 8© Focus Planning Ltd
  • The Completed Forward Pass ©Focus Planning Ltd 9 Below is the completed forward pass, this shows the total duration as 27 days (the Early Finish on Activity 8).
  • The Backward Pass: First Activity The first activity in the backward pass is the last activity in the diagram, as we are now working back through the logic from Finish to Start. As Activity 8 is the last activity the Late Finish will be equal to its Early Finish. To calculate the Late Start we simply take the Late Finish and subtract the duration, so Late Start = Late Finish – Duration. 10© Focus Planning Ltd
  • The Backward Pass: Second Activity Activity 7 has only one predecessor on the backward pass which is activity 8. Because of this relationship Activity 7 has an late finish date equal to the Late Start date on Activity 8. The Late Start of Activity 7 is the same calculation used for Activity 8, which is Late Start = Late Finish - Duration. In this case 25 (LF) - 5 (D) = 10 (LS). Continue to calculate the Late Start and Late Finish for all the remaining activities in the schedule, remembering that in this example will all Finish-To-Start logic that Late Finish = Predecessor Late Finish, and that Late Start = Late Finish - Activity Duration. If there is more than one predecessor then use the earliest Late Start of the predecessors as the Late Finish for that node. 11© Focus Planning Ltd
  • The Completed Backward Pass ©Focus Planning Ltd 12 Below is the completed backward pass, note on Activity 4 the late finish is equal to the late finish on the last node (activity 8), this is because Activity 4 has no successor.
  • Calculate the Float Now that the forward and backward pass are complete, the final step is to calculate the float. This is the time available for delay on an activity that will not delay the project finish date. So activities with zero float will always delay project completion is they finish later than planned, so there are the activities that are classed as Critical. To calculate the float on an activity use the calculation: Early Finish – Late Finish. Complete this for all nodes. 13© Focus Planning Ltd
  • The Completed Critical Path ©Focus Planning Ltd 14 Once we have completed the Float calculations, you can clearly see the critical path as activities with zero float, these have been highlighted in red.
  • Summary So to summarise in order to manually calculate the critical path we perform a forward pass, a backward pass, and calculate the float. Activities with zero float that are logically linked form the critical path. This is the trained approach to critical path analysis used by various organisation and industries, although it is work noting there are some variations to this classic approach. These include calculating multiple paths, and basing the critical path on the chain of activities with the largest total duration. This will be covered in future slides and a copy will be made available on the Focus Planning Ltd website at http://www.focus-planning.com or by contacting info@focus-planning.com ©Focus Planning Ltd 15