Total Slack, Critical Path & Free Slack Project Scheduling Video

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  • 0:03 Definition of a…
  • 0:38 Inputs & Outputs of…
  • 1:42 Types of Scheduling…
  • 2:50 Critical Path…
  • 4:33 Calculating Free and…
  • 6:14 Lesson Summary
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Lesson Transcript
Instructor: Mike Miller
In this lesson you will learn about project scheduling and how to include items such as total slack, critical path, and free slack. You will also learn the outputs of schedule planning.

Definition of a Project Schedule

A project schedule lists all of a project's milestones, activities, and deliverables, each with start and finish dates and sequenced in a manner that allows for efficient execution. The project activities are linked by relationships, also called dependencies.

Project schedules are normally created in a software program, such as Microsoft Projects or Primavera. Another way to create a project schedule is to use a basic methodology called ball and stick, which draws out the schedule using balls for starts and finishes and the stick for durations.

Inputs & Outputs of Project Scheduling

When you sit down to plan your project schedule, you have several factors to consider. The first factor to consider is the scope of the project by answering the question: What are you trying to accomplish? The second factor is to determine your deliverables by answering: What is the end product of your project? You will then use the technique of decomposition to help you divide your project scope and deliverables into smaller, more manageable parts. And, finally, you will use your expert project management judgment to provide input for project activities, durations, and relationships.

Once you have considered and inputted your project scope, deliverables, and decomposition, you will have three reports to help you with your project schedule. You will have an activities list, which is a comprehensive list that includes all activities to complete the project. You will also have a milestone list, which will show you portions of the project that are significant events for the project. And, you will also have a sequencing plan. Once your planning is completed, the activities and milestone lists will be sequenced in a manner to allow for efficient work flow.

Types of Scheduling Relationships

Project schedules fall under four types of scheduling relationships. Before we discuss these relationships, we need to understand two different types of tasks: predecessor and successor. A predecessor task is a task that is followed by another task once the predecessor task is finished. A successor task is one which must wait to start until the predecessor task is completed, or at least started. Now we can discuss the scheduling relationships. They are as follows:

  • Finish to start (FS): This is a logical relationship in which the successor cannot start until the predecessor is completed

  • Finish to finish (FF): This type of activity is one in which the successor cannot finish until a predecessor finishes

  • Start to start (SS): This type of relationship is where a successor cannot start until a predecessor has been started

  • Start to finish (SF): This type of relationship is where a successor cannot finish until a predecessor starts

Please note that you can schedule leads or lags, such as a start to start relationship plus one day.

Critical Path Methodology

Critical path methodology is used to estimate the minimum project duration (what will delay the project's completion) and how flexible the schedule can be. This flexibility is called slack, which is the amount of time that a task can be delayed without pushing the end date of the project. This is also known as free slack or free float. You can have negative float; this is also known as behind schedule. If the task's slack is negative, then the task is behind schedule. If the task's slack is positive, then that is amount of time the task can be delayed prior to delaying the project. For instance, if the task slack is two, then the task can be delayed two hours before it will negatively affect the project. If the task slack is equal to zero, then the activity is on critical path.

Total slack is the amount of time the whole project can be delayed without pushing the end date of the project. This is calculated by using early and late starts and early and late finishes for tasks. Total slack is calculated as the smaller value of the late finish minus the early finish when compared to early start minus the late start.

Total slack can be either positive or negative. If total slack is positive, then it indicates the amount of time the project can be delayed without extending the project. If the total slack is negative, then the project is behind schedule.

Example of a Schedule

Now that we've discussed different ways to view project scheduling, let's look at it in practice. For this example, we will use a portion of building a new construction house. Specifically, we will discuss the roof build. So, we know the scope of the project and our deliverable is a roof successfully installed on the house.

Milestone List

Our milestone list for this project is:

  • Material delivery
  • Ridge cap installed

Activities List with Durations

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