Microsoft Project Work Equation and Task Types

Microsoft Project Work Equation

Introduction

When assigning resources to tasks, Microsoft Project uses a simple formula to calculate how long each task will take, given the specified number of resources. This formula can be written as:

Duration = Work / Units

Where:

Duration

The calculated amount of time it will take to complete the task.

Work

The amount of time (usually expressed in hours or days) that it would take one person to complete the task if they were working at it full time every day.

Units

If a person is said to spend 100% of their time at work then this is the amount of time that they spend working on this project. Units are expressed as a percentage or a decimal. Thus if they are available to spend their whole time working on task then Unit would be 100% or 1.0. If they are only available to spend half their time on a project then Unit would be 50% or 0.5

Thus if it takes one man working full time 8 hrs to build a wall the duration of the task will be:

Duration = work / unit = 8 hrs / 1 = 8 hrs

Duration = 8hrs

Whereas if he is only available to spend 50% of his time on the task

Duration = work / unit = 8 hrs / 0.5 = 16hrs

Duration = 16hours or 2 days

And if there are two men working full time then

Duration = 8 hrs / 2 = 4 hrs

Task Types

Microsoft Project gives us some control over what happens to tasks when we change Duration, Work or Units. There are three task types:

• Fixed Units
• Fixed Work
• Fixed Duration

In these instances the word Fixed means that, of the three variables, Project will keep one constant, so that if we change the second Project will automatically change the third to ensure that the formula remains correct.

For example if a task is Fixed Duration, with a Duration of 8 hours, Work of 8 hours and Unit of 1.0, then we know from the above formula that

Duration = Work / Unit

8 = 8 / 1

If we now change the number of units available to work on the task to 0.5 then the amount of Work needs to change, because it is a fixed duration task.

Since the duration must stay the same at 8hours

8 hours (Duration) = ? hours(Work) / 0.5

Work = 8 (Duration) * 0.5 = 4

Work = 4 hours

Summary

The relationships between the task types and the impact of changing one variable on the other, is shown in the following table.

Task TypeYou ChangeDurationUnitsWork Project will change Fixed DurationWorkWorkUnitsFixed UnitsWorkDurationDurationFixed WorkUnitsDurationDuration