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.
Our milestone list for this project is:
- Material delivery
- Ridge cap installed
Activities List with Durations
|Activity||Duration||Early Start||Early Finish||Late Start||Late Finish|
|Estimate materials||6 hrs.||1/21/16 06:00||1/21/16 12:00||1/21/16 0800||/21/16 1400|
|Order materials||2 hrs.||1/21/16 12:00||1/21/16 14:00||1/22/16 0800||1/22/16 1600|
|Install trusses||2 days||1/23/16 06:00||1/25/16 16:00||1/24/16 0800||1/26/16 1600|
|Install plywood decking||1 day||1/26/16 06:00||1/26/16 16:00||1/26/16 0800||1/26/16 1600|
|Install drip edge||4 hrs.||1/27/16 06:00||1/27/16 10:00||1/27/16 0800||1/27/16 1200|
|Install ice and water shield||4 hrs.||1/27/16 10:00||1/27/16 14:00||1/27/16 1200||1/27/16 1600|
|Install felt||4 hrs.||1/28/16 06:00||1/28/16 10:00||1/28/16 0800||1/28/16 1200|
|Install shingles||2 days||1/28/16 10:00||1/29/16 16:00||1/28/16 1200||1/30/16 1600|
|Install ridge cap||1 day||1/30/16 06:00||1/30/16 16:00||1/31/16 0800||1/31/16 1600|
Calculating Free and Total Slack
To help you manage your schedule and ensure you stay on target, you can calculate your project’s free slack and total slack.
Now, in order to calculate free slack for an activity, you need to perform two calculations and take whichever is greater. This will be the free slack for the activity.
The first calculation is Free Slack (FS) = Late start – early startThe second calculation is FS = Late finish – early finishSo, for our example of activity free float for the roof example we discussed above, our calculations would be:FS = late start – early start = 0800 – 0600 = 2 hrsFS = late finish – early finish = 1600 – 1400 = 2 hrsIn this example, both calculations result in +2 hrs. float. So, float for this activity is +2.
You have two ways to calculate total slack for a project. The first way isn’t always the easy way and sometimes, it’s not very accurate. For example, you have to set up an exhibit and, based on your expert judgment, you know it will take six days to complete. The contract gives you 90 days to complete your project.
This example would be calculated:90 days – 6 days = 84 days of total slack to complete the project.This may not be accurate as projects normally have other constraints that might not be apparent. For our exhibit example, one constraint could be that the space is not available until seven days before the contracted end date.The more preferred way to calculate total project slack is to sum all the activity floats.
In this lesson you learned that a project schedule is a listing of all projects milestones, activities, and deliverables, each with start and finish dates and sequenced in a manner that allows for efficient execution. You also learned about predecessor and successor tasks, and that a predecessor task is a task that is followed by another task once the predecessor task is finished, while a successor task is one which must wait to start until the predecessor task is completed, or at least started.
You also learned that activities can have relationships such as:
- Start to start: where the successor cannot start until a predecessor has been started
- Finish to start: a logical relationship in which the successor cannot start until the predecessor is completed
- Start to finish: where a successor cannot finish until a predecessor starts
- Finish to finish: one in which the successor cannot finish until a predecessor finishes
You also learned that 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. Float, or slack, is the amount of time that either a project or activity can be delayed without extending the project; while total slack is the amount of time the whole project can be delayed without pushing the end date of the project.