Before explaining the slack time and slack time formula, it is necessary to understand the term “Slack” and what does it mean by slack? Generally, the term slack refers to laziness. For example, you may have heard your teacher calling those students slack who are not punctual.
However, in project management, slack is also used in similar manners but in a positive way. It means slack time refers to finishing a project on time.
What is Slack Time? Slack Time Defined in terms of Project Management
Slack Definition: Slack time is referred to as the time difference between the required date to fulfill the critical path and the scheduled completion date.
In simple words, with respect to pert and CPM, slack time is the amount of time that can delay the completion of a task without delaying the other tasks or without delaying the overall deadline of the project.
The “Float” is another term that we use alternatively for “Slack”. Don’t be confused, “Slack” and “Float” are the same things in project management.
Why do we need slack time?
If all the tasks are being finished according to their schedules, then there is no need for slack time. However, when there is a situation where a task demands time more than the estimated time, then the project manager creates slack time to keep the project on time.
For example, if a task requires deep research or more than expected testing, then this task time may delay the project deadline.
To keep the deadline intact, the project manager analyses the time required by all the tasks individually. Then the manager allocates the extra time to this task from another task that may task less time than the allocated time.
In this way, slack time helps keep the project on time which would have been delayed if the project manager had followed the allocated time for each task.
On the other hand, if you, as a project manager, go over the slack time, then it can affect the time of other tasks and the overall project deadline as well.
Importance of Slack Time in Project Management
It is crucial to finish projects on time when it comes to the larger firms and brands. This is the reason that project managers keep the extra budget in their back pockets to balance the imbalance situations to meet the project deadlines when there is the possibility of slack time occurrences.
In this case, we must calculate the slack time for all the activities on the PERT chart. If ignored, it can lead to some serious results like project deadlines can’t meet.
Not only it affects the project’s deadline, but also it can have a bad impact on the efficiency and performance of your team.
The following are the points that reveal the importance of slack time in project management.
- Slack time helps finish the project on time when there are time imbalances in the individual tasks of the project.
- Slack time helps to estimate the time available between the various steps of a project.
- More time can be allocated to critical activities to ensure the project is delivered on time.
- Slack time can help shorten the PERT chart path to improve productivity. But we must do it carefully, as shortening the paths can increase project cost if an activity takes more than expected time.
- Everyone knows how much time he/she has to work on a certain task of a project which ensures the project is delivered on time. Not only slack time keeps the project on time, but also keeps it within the allocated budget.
- Slack time also helps prioritize the tasks to improve the team and the project’s efficiency. It means the activities with zero slack must be at the top of your to-do list.
- Slack time loosens up the stressed and burned environment. As a result, the overall efficiency of the team improves.
Suppose you have to finish a task today but the task is dependent on another task that will be finished by another person but not today. What will happen? The uncertain delays.
That’s why slack time management is the most important responsibility of the project manager.
Slack Time in PERT (Program Evaluation and Review Technique) Network
Note: Slack time can only be used for those tasks that don’t lie on the critical path of the PERT chart. Critical tasks that must complete on time to meet the project deadline, must not take any slack time. Such tasks must be completed within their allocated time.
However, the earliest start time and the latest start time will be the same for all tasks on the critical path. The same applies to the earliest finish time and the latest finish time.
Information: A critical activity or task is the activity that has zero as the slack time.
We have used the terms “Earliest Start Time”, “Latest Start Time”, “Earliest Finish Time”, and the “Latest Finish Time”, it is better to explain these terms now because we will be using them later in this article.
Before explaining these terms, suppose you have a project that has 3 activities/tasks (Task A, Task B, and Task C) and each task takes 5 days to finish if each task is finished on its time.
Also Read in Details: Program Evaluation & Review Technique
Earliest Start Time (EST):
The earliest start time is the time or date when an activity starts as early as possible after its predecessors finish before their estimated finish time.
From the previous example, if task A finishes just in 3 days instead of 5 then task B starts 2 days before its actual start time. In this case, we call these 2 early days the earliest start time.
In this case, all the successor activities of the EST activity start at their earliest time because one EST activity affects all the upcoming activities. Similarly, all successors finish at their earliest finish time.
Latest Start Time (LST):
The latest start time means the time when an activity can start as late as possible without impacting the deadline of the project. This happens when an activity starts at its late start time and all the successors of that activity become LST activities.
Earliest Finish Time (EFT):
The earliest finish time means the time when an activity finishes as early as possible. In this scenario, all the successor activities become EFT activities. Similarly, all successors start at their earliest start time.
Latest Finish Time (LFT):
The latest finish time is the time when an activity finishes as late as possible without affecting the overall project deadline.
In this case, the successor activities start at their latest start time and finish at their latest finish time.
Let’s now understand slack time in the PERT network.
Event 4 is not on the crucial path in the following figure.
Note: It is necessary to remember that there occur only a single longest path in the network and the other paths are being shorter than that length or equal to that length. Therefore, the activities and events should finish before the actual required time.
Seven weeks are required to go from event 2 to event 5 on the critical path by taking the route 2, 3, 5. Only four weeks are needed if the route 2, 4, 5 is adopted. Therefore three weeks are required if route 2-4-5 is adopted.
Therefore event 4 should begin anywhere between zero to three weeks because it needs two weeks for completion after event 2 is completed. Another use for resources of people, facilities, money, and equipment needed to finish event 4 must be searched by the management during these three weeks.
Therefore for resource allocation and scheduling as the critical path is important, because those events that are not on the critical path for achievement can be rescheduled by the project manager with the coordination from the functional manager, during other time periods when maximum utilization of resources can be accomplished under the provision that the critical path time is not extended.
A better balance of resources throughout the company is provided by this type of rescheduling through the utilization of slack times. Even by eliminating waiting or idle time, project costs may possibly reduce.
Slack Time Formula
We can calculate the slack time easily for each activity by using a simple formula which is given below.
This formula indicates that the slack time is equal to the difference between the latest permit able date and the earliest expected date based on the following nomenclature is said to be slack. Here is the formula.
Slack Time = LST – EST
Where LST stands for “Latest Start Time” and EST stands for “Earliest Start Time”.
This formula is also known as Activity Slack Formula. Here is the “Activity Slack” has the same meaning as the slack time. It means the activity slack also refers to the length of the time an activity can be delayed without delaying the project deadline.
Another formula for calculating slack time is to subtract the latest finish time from early finish time which is in the form of the formula:
Slack Time = LFT – EFT
Where LFT stands for “Latest Finish Time” and EFT stands for “Early Finish Time”.
In simplest words, the slack time is the difference between the time allocated to the task and the time it actually takes.
For example, if you allocate 20 days to a task but the task is finished in 15 days then the slack or float time is 5 days.
How to Use Slack Time Formula?
Let’s understand the slack time formula with an example.
Suppose Christina is a chef at a famous five-star hotel and she is assigned a project on 15th October to introduce a completely new recipe for the customers. She is given the deadline of 15th November.
Before she prepares the recipe, she has to collect the ingredients that meet the specifications of the recipe. So, her first task is to collect ingredients with certain nutritional values.
Now Christina starts to calculate the slack time for her first task which is collecting ingredients. The deadline for this task is 2nd November. Christina finds that the earliest start time is the 10th of October and the latest start time is the 25th of October.
Christina calculates the slack time of this task like this:
Slack Time = 25th October (LST) – 10th October (EST)
So, the slack time for collecting ingredients with specific nutrition is 15 days. It means Christina has 15 days to finish this task. This is a simple slack time example that explains this term well.
How to Calculate Slack in Project Management?
The calculation for slack time is conducted for every event in the network as shown in the following figure by recognizing the latest starting date and the earliest expected date. TL – TE = 0 for event 1.
The reference point for the network can be served as event 1 and can be easily specified as a calendar date. The bold line represents the critical path as before. There is no slack (i.e. TL = TE) for the events on the critical path and boundaries for events of the non-critical path is also provided.
For event 5 TL = TE x 3 + 7 = 10, since event 2 is important one. The critical path is terminated by event 6 with a completion time of 15 weeks.
The earliest time for event 3 would be two weeks (TE = 0 + 2 = 2), which is not on the critical path considering that it started as quickly as possible. By subtracting the time needed to finish the activity from events 3 to 5 from the latest beginning date of event 5 the latest allowable date is acquired.
Due to this fact, TL (for event 3) = 10 – 5 = 5 weeks. Now event 3 may happen anywhere between weeks 2 and 5 without disturbing the planned finishing date of the project. Event 4 can be applied the same procedure in which situation TL = 9 and TE = 6.
A simple PERT network is included in the above-mentioned diagram and therefore slack time calculation is not much difficult. The earliest beginning dates should be ascertained by proceeding from start to completion through the network, for complex networks including multiple paths.
Similarly, by working backward from completion to beginning, the latest allowable beginning date can be calculated.
It should be cleared that the significance of recognizing clearly where the slack persistence cannot be overstated. Better technical performance is permitted by the effective use of slack time. It is finding by Donald Marques that organizations that effectively employ slack time were 30% more successful than the average in accomplishing technical needs.
Because of slack times, PERT networks are mostly not plotted with a time scale. There can be a requirement for the planning requirements that the PERT charts be rebuilt with time scales, in which situation a decision should be taken either to wish late or early time requirements for slack variables.
The above figure represents this by comparing manpower planning with total program costs. In this figure, early time requirements for slack variables are used.
The profitability of successfully meeting the schedule is ascertained by combining the earliest time and late time. In the table below, a sample of the needed information is highlighted.
The latest and earliest times are regarded as random variables. The actual schedule indicates the schedule for event occurrences that were built at the start of the project. In this table, the last column provides the profitability regarding the earliest time that will not surpass the actual schedule time for a specific event.
In the example represented above, for every event, the latest times and earliest times were calculated. The earliest and latest times were preferably evaluated for every activity instead.
Additionally, the latest and earliest times were recognized merely as the date or time when an event may be anticipated to occur.
The following four values need to be identified to make full use of the capabilities of the PERT Analysis / CPM.
- The earliest time when an activity may start (ES)
- The latest time when an activity may start (LS)
- The earliest time when an activity may finish (EF)
- The latest time when an activity may finish (LF)
The earliest and latest times recognized on the activity are shown in the following figure.
A forward pass through the network should be made to evaluate the earliest starting times (i.e. left to right). The latest of the earliest finish dates of the predecessor activity is actually the earliest starting time of a successor activity. The total of the earliest starting time & activity duration is the latest starting time.
A backward pass through the network should be made by evaluating the latest finish time to calculate the finishing time. The latest starting time can be evaluated by subtracting the activity time from the latest finishing time since the activity time is known. The earliest finishing time of activities exiting the node is the latest finishing time for an activity entering a node.
For a typical network, the earliest & latest starting & finishing times are shown in the following figure.
For the project manager, slack identification is quite significant as it functions as an early warning system. For example, if the available total slack time starts to diminish from a specific reporting duration to the next that might point out that more highly skilled labor is required or longer than forecasted time is taken by the work. A new critical path might be building.
Slack time can be identified by looking at the earliest and latest start & finish times. Below is an example of the two cases that should be considered.
The slack time is easily recognized as four work units in situation a, where the work units can be expressed in months, weeks days, or hours. This is said to be negative float or negative slack.
To understand the answer to the question that what can cause the slack to be negative. The following figure is helpful for this purpose.
Work is done from left to right beginning at the customer’s starting milestone (position 1) when conducting a forward pass through a network. The backward pass, on the other hand, does not starts where the forward pass ends but at the customer’s end date milestone (position 2). It is possible to have slack on the critical path if the forward pass ends at position 3 which is before the customer’s end date.
The slack time is mostly referred to as reserve time and may be filled with other activities or combined with activities like report writing to extend the forward pass to the customer’s completion date.
It is important to note that when the forward pass extends beyond the customer’s end date, negative slack normally occurs as represented by position 4 in the above diagram. However, the customer’s completion date is still used to measure the backward pass, thus forming negative slack.
This is probably to result when:
- The actual plan was very optimistic but unrealistic
- During project execution, one or more activities slipped
- The correct skill level has not been possessed by the assignment resources
- The customer’s end date was not realistic
- Until a later date, the needed resources would not be available
Negative slack is an early warning indicator in any event. The customer’s end date is required to be maintained as corrective action indicated by the early warning indicator.
PERT Charts and Slack Time
Although, PERT charts are not the topic of this article, let’s have a quick look at pert charts slack time. However, I will cover the details of PERT charts in another article. Keep visiting BusinessStudyNotes.
When it comes to PERT charts, we can use an activity card for each activity to create a PERT network. Each activity card must include the full details of the activity, especially the information related to slack time.
The PERT activity card must include:
- Duration: Total time required to finish the activity
- Latest Start Time (LST)
- Latest Finish Time (LFT)
- Earliest Start Time (EST)
- Earliest Finish Time (EFT)
- Slack Time (ST = LST – EFT)
Note: The early start time (EST) of an activity card will be the early finish time (EFT) of the previous activity. Similarly, the latest start time (LST) of the activity will be the latest finish time (LFT) of the preceding activity.
However, if an activity has multiple preceding activities and it can start only when the previous activities finish then, in this case, use the finish time of the previous activity that finishes at the end as the start time of the current activity.
After filling out the PERT chart with the slack time activity card, it may look like the following PERT chart.
Here is the latest start time (LST) is the difference between duration and the latest finish time of the activity. Here is the formula:
Latest Start Time (LST) = Duration (total time of the activity) – Latest Finish Time (LFT)
In the above PERT chart, the critical path is the path where all the activities have zero slack time.
Note: Any delay to any activity of the critical path may result in the delay of the project’s deadline.
Slack Time Tracking
By now, you have complete knowledge of slack time and the slack time formula. Now, it’s easy to track slack time. Tracking doesn’t involve any complex activities. All you need to know are two dates or times.
- Earliest Start Date
- Latest Start Date
Then using the slack time formula, calculate the float time. This is how we can track the slack time.
Free Slack vs Total Slack
2 main types of slack or float are the following.
- Free Slack: The amount of time a task can delay without delaying the deadline of succeeding tasks is called free slack.
- Total Slack: The amount of time a task can delay without delaying the deadline of the project is called total slack. Total slack is sometimes called “Slack” or “Float”.
How to Calculate Free Slack?
As the free slack is related to the individual tasks, so it can be calculated by taking the difference between the next sequential task’s estimated start date and the end date of the current task.
The free slack formula is:
Free Slack = EST of next activity – EFT of current activity
Where EST means early start time and EFT means early finish time.
How to Calculate Total Slack?
We calculate total slack by taking the difference between the finishing date of the last task on the critical path and the expected finishing date of the task for which we are trying to calculate total slack.
The total slack formula is:
Total Slack = Latest Start Time – Earliest Start Time
Slack Time Summary
In simple words, Slack Time is the amount of time certain tasks of a project can delay without hurting the overall deadline of the project. In bigger projects, it is crucial to calculate and assign slack time to those smaller tasks that are less-time consuming so that the major and most important tasks can have enough time to be finished on time.
We calculate the slack time for any project by subtracting the earliest start time from the latest start time for that certain task. Slack time for an activity in a PERT chart can be calculated by the following simple formula which is:
Slack Time = Latest Start Time – Earliest Start Time
1. What is float in project management?
Float or slack, are the same things in project management. Slack time is the amount of time a task can delay without delaying the project deadline.
2. How to determine slack in a project? Or how to find slack time?
We calculate the float or slack time of activity by taking the difference between the latest start time and the earliest start time. The formula to calculate slack time is:
Slack Time = Latest Start Time – Earliest Start Time
3. What is critical path in project management?
The simple definition of the critical path in project management is the path on the PERT chart where all the activities have zero slack time. It means all the activities on the critical path must start and finish within the allocated time. Otherwise, it may delay the project deadline and may increase the project budget.
4. Why is slack important to the project manager?
Calculating slack time is important for project managers because it helps them prioritize the most important tasks. It also helps to allocate resources more efficiently while keeping the project delivery on time.
5. What is activity slack?
Activity slack is the same as the slack time. It represents the time an activity can delay without delaying the project’s overall duration.
6. How to calculate slack in critical path?
We don’t calculate slack on the critical path because each task/activity on the critical path has zero slack. It means we can’t delay tasks on the critical path by any means.
7. What does PERT mean?
PERT, in project management, stands for Program Evaluation Review Technique. It is a project management tool used to calculate the amount of time a project is going to take to be finished. Generally, we use PERT charts in CPM (critical path method).
It means PERT is a graphical representation of the project’s timeline. In this tool, we divide the project into smaller tasks and then the completion time for each task is calculated separately.
8. How to calculate slack time in network diagram?
Calculating slack time in the network diagram or PERT chart tool is the same where we need the latest start time and earliest start time of the activity we are calculating the slack time for.
9. Can slack time be negative?
Yes, there are situations when the slack time can go negative. A positive slack time means the project has flexible time that can be allocated to individual tasks to finish the project before the deadline.
However, negative slack time means the project has a very tight deadline and there is a chance that the project will be delayed. This negative number indicates the amount of time that the project manager must save to finish the project on time.
10. Is there any slack time calculator available?
Unfortunately, there is no such calculator available because the calculation of slack is very simple. All you need is the latest start time and earliest start time of activity to calculate the slack time of that activity. Then putting these values into the slack time formula gives the result.
11. How to calculate early start and early finish?
The early start of an activity is the highest early finish time value of the previous activity. If the activity is the first one in the slack chain then the early start is the time when the activity can start as early as possible.
We calculate the early finish of an activity as the sum of the early start and the total time, the activity is going to take.
EF = ES + Duration
12. How to identify critical path?
The simplest way to find the critical path in CPM is to find the path where all the activities have zero slack.
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