Most Project users expect it to act like a spreadsheet and are often surprised to find that Project appears to have a mind of its own. This is because Project operates within an elegant framework of rules, algorithms, and solid project management techniques. The key to understanding this framework is a simple formula, which we will call the Basic Work Formula. Starting from complete basics, let's now proceed to understand this formula and see how it drives complex scheduling behavior.
There are three key parameters associated with every task to be executed in a schedule:
work is the effort required to finish a task, usually measured in person-hours.
duration is the span of working time required to finish a task, usually measured in working days.
Unit is the mesaure of resoures allocated to a specific task, measured by percentage in Project.
These three parameters are related by the Basic Work Formula of Microsoft Project:
Duration = Work / Units
Using simple algebra, it can also be written as follows:
Work = Duration x Units
Alternatively, it can be written as follows:
Units = Work / Duration
Now, let's really cement our understanding of the Basic work Formula by considering some simple examples. Let's say our testing team must execute 80 automated test cases for the company's software. One engineer can execute one test case in 1 hour. How long does the whole task take?
From the preceding problem statement, we know the following:
work = 80 person-hours (because one test case can be executed in hour 1)
Units = 1 (because we have one test engineer working 100% on this project)
We can now compute this by applying the Basic work Formula:
duration = 80 person-hours / 1 Unit (this is the Basic Work Formula)
duration = 2 work weeks (which is 80 person-hours)
Now, just to make this more interesting, let;s say a duration of 2 work weeks is not acceptable to you, and you want to reduce this time further. You decide to speed things up by adding one more test engineer to the task.
Now, we have the following:
work = 80 person-hours (this has not changed)
Units = 2 (because we now have two test engineers working 100% on this project)
We can now compute this again:
duration = 80 person-hours / 2 Units (this is the Basic Work Formula)
Therefore, duration = 1 work week (which is 40 person-hours)
By doubling the number of resources, we have halved the duration
This isn't exactly rocket science but let's continue with yet another example.
A third test engineer has become available to work on your project. But they can only work 50%of their time on your project. Now, your Units is 2.5 (which is the same as 250% within Project) and, of course, work is the same at 80 person-hours. What will the new duration of your task be? Take a moment to compute the answer by applying the Basic Work Formula. You should get an answer of 4 workdays (on 32 hours)
chanelcolgate
commented
2 years ago 
Most Project users expect it to act like a spreadsheet and are often surprised to find that Project appears to have a mind of its own. This is because Project operates within an elegant framework of rules, algorithms, and solid project management techniques. The key to understanding this framework is a simple formula, which we will call the Basic Work Formula. Starting from complete basics, let's now proceed to understand this formula and see how it drives complex scheduling behavior. There are three key parameters associated with every task to be executed in a schedule:
Basic Work Formulaof Microsoft Project:Using simple algebra, it can also be written as follows:
Alternatively, it can be written as follows:
Now, let's really cement our understanding of the Basic work Formula by considering some simple examples. Let's say our testing team must execute 80 automated test cases for the company's software. One engineer can execute one test case in 1 hour. How long does the whole task take? From the preceding problem statement, we know the following:
We can now compute this by applying the Basic work Formula:
Now, just to make this more interesting, let;s say a duration of 2 work weeks is not acceptable to you, and you want to reduce this time further. You decide to speed things up by adding one more test engineer to the task. Now, we have the following:
We can now compute this again:
Therefore, duration = 1 work week (which is 40 person-hours) By doubling the number of resources, we have halved the duration This isn't exactly rocket science but let's continue with yet another example. A third test engineer has become available to work on your project. But they can only work 50%of their time on your project. Now, your Units is 2.5 (which is the same as 250% within Project) and, of course, work is the same at 80 person-hours. What will the new duration of your task be? Take a moment to compute the answer by applying the Basic Work Formula. You should get an answer of 4 workdays (on 32 hours)