Leads, lags and float are concepts used in schedule development process. The process of schedule development includes: identification of all activities, sequencing all activities based on dependency, estimating duration of each activity and finalizing the schedule. Leads, lags and float are used as part of activity sequencing process. All activities once sequenced will form a schedule network diagram. Let us first look at the definition of all these three attributes. Show
FloatFloat (also known as slack) is the amount of time by which the start of an activity can be delayed without delaying the project completion time. Every task will have following set of start and finish time.
Float time of an activity can be calculated by taking the difference between Late Start (LS) and Early Start (ES) OR between Late Finish (LF) and Early Finish (EF). Float = LS-ES OR =LF-EF A positive float time indicates the flexibility we will have in delaying the specific activity without delaying the project completion time. Typically, while doing scheduling, the critical path tasks will have zero float and the non-critical path tasks will have a positive float. That means non-critical path tasks can be delayed to certain extent without compromising on the project completion time. Float time information of tasks is very useful to the project team for taking scheduling decisions when there will be resource constraints. Lag:Lag is the amount of wait time between two tasks. Or in other words, lag is the amount of time by which a successor activity will be delayed. Lag can be used in all the four logical relationships in scheduling, such as Finish-to-start (FS), start-to-start (SS), finish-to-finish (FS) and start-to-finish (SF). In below example, Task A and B have a Finish to Start (FS) relationship. Ideally both A and B should get finished on the 12th day. But when we insert a waiting of time of 2 days before B can start, then both A and B will get completed only on the 14th day. Lead:Lead is the amount of time a successor task can be accelerated. Lead can applied only on finish-to-start relationship between two activities. We can see the below example. In the below example, task B can start 2 days before the completion of task A. Hence the start of task B, which ideally would have been on 6th day, will not start on 4th day. Conclusion: Float, lead and lag are very important concepts and information for the scheduling team. A PMP training course ensures you get a hold of these concepts. These are used to optimally identify the dependencies and the associated constraints. Float information is useful in resource allocation when there are resource constraints. Lead is used for accelerating start of tasks (fast tracking) for reducing project timelines. Lag is used for ensuring that required idle or wait time after a task is appropriately provisioned. Critical Path Method (CPM) is a project schedule modeling technique. Mr. Morgan R. Walker and James E. Kelly developed this technique in the late 1950s. Project planners use this method to develop schedules for projects, including IT, research, and construction. The critical path method is a lengthy and complex concept. Please follow each step in this blog post, and don’t move on until you understand the previous steps. If you follow this advice and complete the blog post, you won’t have any problems solving Critical Path Method questions on the PMP Exam. Critical Path MethodIn the critical path network method, you focus on managing the critical path. A network diagram has many paths originating from one point and ending at another point. Every path has a duration, and the one with the longest duration is the critical path. You can define a critical path as:
Notice that the first statement talks about the longest path, and the other talks about the shortest duration. They may appear to be opposites, but they are conveying the same message. For example, let’s say you have a project to construct three buildings. The first is the largest, the second is medium-sized, and the third is the smallest. You develop the network diagram, which comprises three paths, each representing a building. You calculate the duration for each path. For the first building, the duration is 31 months, the second will take 18 months, and the third will require 13 months. You can see that the first path is for the largest building, the second path is for the medium-sized one, and the third path is for the smallest building. Now, let us review the diagram for the critical path analysis. Did you notice that the first path is the longest? It is 13 months longer than the second and 18 months longer than the third. This means that you can wait 13 months and then start working on the second building because you can complete the second building in 18 months. Likewise, you could wait 18 months and then start working on the third building because it will take 13 months to complete. This means that, even if you start work on the third building after 18 months from the project start date, you can finish it on time. This waiting period is known as the float or slack. So, which is the critical path in this network diagram? It is the longest path on the network diagram because you cannot complete your project before finishing the first building. Although you can complete the other two buildings quickly, your project is not considered complete until the first building is completed. This confirms the first statement that says, “the critical path is the longest path on the network diagram.” Now, what is the shortest duration to complete the project? It is 31 months because you cannot complete your project in less time, and this is the duration of the critical path. This bears out the second statement that says, “The critical path is the shortest duration in which you can complete the project.” So, both definitions are the same. You can define the critical path as the sequence of activities from start to end, and it has the longest duration of all paths in a network diagram. In ideal conditions, a network diagram should have one critical path. Multiple critical paths will put you in a difficult situation. The critical path has the longest duration, and it is the total project’s duration. Activities on the critical path have no float; therefore, you must ensure those critical activities are completed on time. Any delay in a critical activity (critical path activity) will delay the project. What if the project is delayed? Schedule slippage is common in project management. However, some tools can help you bring things back on schedule. These are called schedule compression techniques, and Fast-tracking and Crashing are two examples. If your project is behind schedule, you can use these tools to get it back on time. Visit: Fast Tracking and Crashing A Few Commonly Used Terms in Critical Path MethodBefore I start discussing how to find the critical path, let us understand a few common terms used in a critical path network diagram: Earliest Start Time or Early Start (ES): This is the earliest time an activity can be started in your project. Latest Start Time or Late Start (LS): This is the latest time that an activity can be started on your project. If you start the activity beyond this time, it will affect your critical path. Earliest Finish Time or Early Finish (EF): This is the earliest time an activity is completed in your project. Latest Finish Time of Late Finish (LF): This is the latest time you can complete the activity on your project. If your activity crosses this time, your project will be delayed. How to Find the Critical Path in a Network DiagramNow we have reviewed critical path method terminology, let’s see how we can calculate the critical path, late start, early start, late finish, and early finish:
This information lets you draw the critical path network diagram, and you will know the float of each activity. You will how much you can delay a particular task, and any delay beyond this point will affect the project schedule. Critical Path Method ExampleBased on the network diagram below, identify the total paths, critical path, and float for each path. The above network diagram has five paths. The paths and their durations are as follows:
Since the duration of the first path is the longest, it is the critical path. The float on the critical path is zero. The float for the second path “Start ->D -> E ->F -> End” = duration of the critical path – duration of the path “Start ->D -> E ->F -> End” = 31 – 18 = 13 Hence, the float for the second path is 13 days. Using the same process, we can calculate the float for other paths as well. Float for the third path = 31 – 26 = 5 days. Float for the fourth path = 31 – 13 = 18 days. Float for the fifth path = 31 – 16 = 15 days. Calculate Early Start, Early Finish, Late Start, and Late FinishWe have identified the critical path and the duration of the other paths. Now it’s time to move on to more advanced calculations: Early Start, Early Finish, Late Start, and Late Finish. Calculating Early Start (ES) and Early Finish (EF)To calculate the Early Start and Early Finish dates, we use the forward pass; we will start from the beginning and proceed to the end. The Early Start (ES) for the first activity on any path will be 1 because you cannot start the activity before the first day of your project. The starting point for any activity is the endpoint of the predecessor activity on the same path (plus one). The formula used for calculating Early Start and Early Finish dates:
Early Start and Early Finish Dates for the path Start -> A -> B -> C -> EndEarly Start of activity A = 1 (Since this is the first activity of the path) Early Finish of activity A = ES of activity A + activity duration – 1 = 1 + 10 – 1 = 10 Early Start of activity B = EF of predecessor activity + 1 = 10 +1 = 11 Early Finish of activity B = ES of activity B + activity duration – 1 = 11 + 12 – 1 = 22 Early Start of activity C = EF of predecessor activity + 1 = 22 +1 = 23 Early Finish of activity C = ES of activity C + activity duration – 1 = 23 + 9 – 1 = 31 Early Start and Early Finish Dates for the path Start -> D -> E -> F -> EndEarly Start of activity D = 1 (Since this is the first activity of the path) Early Finish of activity D = 1 + 5 – 1 = 5 Early Start of activity E = EF of predecessor activity + 1 Since activity E has two predecessor activities, which one will you select? The answer is the activity with the greater Early Finish date. The Early Finish of activity D is 5, and the Early Finish of activity G is 3 (we will calculate it later). Therefore, we will select the Early Finish of activity D to find the Early Start of activity E. Early Start of activity E = EF of predecessor activity + 1 = 5 + 1 = 6 Early Finish of activity E = 6 + 7 – 1 = 12 Early Start of activity F = 12 + 1 = 13 Early Finish of activity F = 13 + 6 -1 = 18 Early Start and Early Finish Dates for the path Start -> G -> H -> I -> EndEarly Start of activity G = 1 (Since this is the first activity of the path) Early Finish of activity G = 1 + 3 – 1 = 3 Early Start of activity H = 3 + 1 = 4 Early Finish of activity H = 4 + 4 – 1 = 7 Early Start of activity I = 7 +1 = 8 Early Finish of activity I = 8 + 6 – 1 = 13 Calculating Late Start (LS) and Late Finish (LF)We have calculated the Early Start and Early Finish dates of all activities. Now it is time to calculate the Late Start and Late Finish dates. The Late Finish date of the last activity on all paths will be the same because no activities can continue once the project is completed. The formula used for Late Start and Late Finish dates:
To calculate the Late Start and Late Finish, we use the backward pass; i.e. we will start from the last activity and move back towards the first activity. Late Start and Late Finish Dates for the path Start -> A -> B -> C -> EndOn a critical path, the Late Start, and Late Finish dates will be the same as the Early Start and Early Finish dates Late Start and Late Finish Dates for the path Start -> D -> E -> F -> EndLate Finish of activity F = 31 (because you cannot allow any activity to pass the project completion date) Late Start of activity F = LF of activity F – activity duration + 1 = 31 – 6 +1 = 26 Late Finish of Activity E = LS of successor activity – 1 = LS of Activity F – 1 = 26 – 1 = 25 Late Start of Activity E = LF of activity E – activity duration + 1 = 25 – 7 + 1 = 19 Late Finish of activity D = LS of successor activity – 1 If you look at the network diagram, you will notice that activity D has two successor activities, B and E. So, which activity would you select? You will select the activity with the earlier (least) Late Start date. Here, the Late Start of activity B is 11, and the Late Start of activity E is 19. Therefore, you will select activity B, which has the earlier Late Start date. Hence, Late Finish of activity D = LS of activity B – 1 = 11 – 1 = 10 Late Start of Activity D = LF of activity D – activity duration + 1 = 10 – 5 + 1 = 6 Late Start and Late Finish Dates for the path Start -> G -> H -> I -> EndLate Finish of activity I = 31 (because you cannot allow any activity to pass the project completion date) Late Start of activity I = 31 – 6 + 1 = 26 Late Finish of activity H = 26 – 1 = 25 Late Start of activity H = 25 – 4 + 1 = 22 Late Finish of Activity G = 19 – 1= 18 (we will choose the late start of activity E, not activity H because the Late Start of activity E is earlier than the Late Start of activity H). Late Start of activity G = 18 – 3 + 1 = 16 Calculate the Free FloatI recommend that you read my blog post on the total float and free float to get a better understanding before proceeding further. Visit: Total Float and Free Float The formula for the Free Float is:
Benefits of the Critical Path MethodThe following are a few benefits of the Critical Path Method:
Drawbacks of the Critical Path MethodAlthough the critical path is a very useful tool in project planning, it has some drawbacks, such as:
To overcome these shortcomings, the Critical Chain Method (CCM) was developed. Visit: Critical Chain Method in Project Management Apart from the CCM method, many project planners use schedule based on the Gantt chart. SummaryThe Critical Path Method has helped many project managers develop and manage schedules. It is a good communication tool, and it helps secure stakeholder buy-in. A network diagram has many paths, but you must focus on the critical one. Any delay in critical activity will affect the project schedule. Monitor floats on other paths because if the float drops to zero, that path will become a critical path, and you should avoid this. As a project manager, you must monitor your network diagram and take prompt corrective action whenever necessary. Are you involved with project planning? Please share your experience with the Critical Path Method in the comments section. This is an important topic from a PMP exam point of view. You will see many questions in your exam on this topic. What is the amount of time that a task can be delayed?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. 2.
What is the time by which it is possible to delay the completion time of an activity without affecting the total project duration?Total Float (TF)
It is the amount of time by which an activity can be delayed without delaying the project duration.
What is the amount of time an activity may be delayed before it delays the project end date?Float, sometimes called slack, is the amount of time an activity, network path, or project can be delayed from the early start without changing the completion date of the project.
What is the amount of time that a task can be delayed without affecting the subsequent task?In project management, float or slack is the amount of time that a task in a project network can be delayed without causing a delay to: subsequent tasks ("free float") project completion date ("total float").
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