Slack time, used in Program Evaluation and Review Technique (PERT), denotes how much an activity can be delayed beyond its earliest start date, without causing any problems in the completion of the project by its due date. Also known as float, slack time is applicable only to those activities which do not lie on the critical path of the PERT chart. For all activities that lie on the critical path the earliest start time will be the same as latest start time and the earliest finish time will be the same as latest finish time. This article will show you how to calculate slack time for your projects. [caption id=“attachment_133264” align=“aligncenter” width=“640”] Show Calculating Slack TimeCalculating the slack time for an activity in a PERT chart is very easy and does not involve any complex calculations. There are two things that are needed to calculate the slack time:
The slack time formula is: Slack Time = LS-ES [caption id="" align=“aligncenter” width=“600”] Alternatively, slack time can also be calculated as the difference between the latest finish time and the early finish time. Slack time must be calculated for all activities involved in the project. Calculating the slack time is another way of tracing the critical path on a PERT chart. All activities which return a ZERO as the slack time, are the critical activities. Screenshot taken by Sidharth Thakur The Importance of Slack TimeSlack time is a very integral part of the project process and it must be clearly shown on the PERT chart. Ignoring slack time can lead to consequences that may compromise the overall performance and efficiency of the project. Only when the slack times have been identified will it be possible to use them to the project’s advantage. Activities which have some slack time can be delayed if resources need to be diverted elsewhere for critical activities. Also at times, devising strategies to reduce slack time brings about an overall improvement in the productivity. The process of allocating resources to shorten the critical path and thus remove or reduce slack time from the PERT chart is termed as crashing. However, crashing a project should be done with caution as it is a cost intensive way of speeding up the process and involves heavy overhead costs, which could radically affect the total project budget. PERT ChartsWhen drawing the PERT activity chart, use an activity card that shows all the time details for an activity, like the one shown in the screenshot above. The time details that all activity cards must contain are:
To start filling in the time details into the activity card, begin from the first activity and fill in the early start and early finish time for each activity. The early start time for the subsequent activities will be the early finish time of the preceding activity. In case there are too many preceding activities, use the finish time of the activity which ends after all the others. The next thing needed to calculate slack time is to determine the latest start and finish times for each activity. For this, the calculations will begin backward, from end toward start, and the maximum time needed to complete the entire project should be used as the late finish time for all the final activities. The latest start time can be calculated by subtracting the duration for each activity from its latest finish time. Where there are two or more successor activities, use the smallest late start time from among all of the activities, as the latest finish time. [caption id="" align=“aligncenter” width=“600”] Finally, to calculate the slack time for each activity, either subtract late start from early start or subtract late finish from early finish. Put in the slack time in the space provided in the activity card. To find out the critical path for the PERT chart, just follow the path of the activities where the slack time is zero. Screenshots taken by Sidharth Thakur You’ll be glad you took the time to find out how much slack you have if you run into a scenario where you are falling behind schedule and more resources are needed on the critical path. Image by Icons8_team from Pixabay What does the slack time mean?Slack time can be defined as the amount of time a task can be delayed without causing another task to be delayed or impacting the completion date of your project.
How is the slack time for an activity is defined in CPM?A float (or slack) in a critical path method (CPM) is the amount of time that a task can be delayed without causing any delay to Subsequent tasks and project completion date.
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