Working with Microsoft Project, have you ever come across one of these "tear your hair out" moments? You're working with Project tasks, and you've just changed how many staff you've assigned to a particular task because you've had a rethink or perhaps you need to deploy staff in a different way. What does Project do - it changes the task duration! After all the work you did figuring out the best task duration in the first place.

The answer lies of course in a Project feature called EFFORT DRIVEN. It works this way. Suppose you have a two task project, "give a presentation" and "analyse feedback forms". The presentation itself is to take four hours to deliver, so you set its task duration to 4 hours. You then assign Fred to give the presentation. Project now calculates the WORK to be done on the task as 4 hours times 1 resource unit (Fred), equals 4 HourUnits. Project doesn't worry about what kind of resource you've assigned to the task, just how many are initially assigned.

EFFORT DRIVEN means that if you subsequently change the allocated resources, Project will keep the total WORK for the task at the original level set when the first resource was assigned. So if you assign Mary to help Fred give the presentation, there are now two resource units assigned to the task. So in order to keep the total WORK at 4 HourUnits, Project changes the task duration to two hours, so the WORK is still 2hr times 2 resource units, equals 4 HourUnits. However our presentation duration must be 4 hours, So you manually change the task duration back to 4 hours. Of course the WORK to complete this task is now increased to 8 HourUnits.

The second task "analyse feedback forms" is done immediately after the presentation in the same room and involves collecting a completed audience questionnaire and then counting how many particular survey questions were ticked. You expect this to take two hours and you assign Jane to do this work. So The WORK for this task is 2hrs times 1 resource unit making 2 HourUnits.

You might want to get this analyses work done as quickly as possible as you want to vacate the room as soon as you can and all head off home, so you assign Bill to help Jane. Once you assign Bill to the task Project applies the same EFFORT DRIVEN principle. So in order to keep to the original WORK at 2 HourUnits it reduces the task duration to 1 hour. So now the WORK is still 2HourUnits - 1 hr times 2 resource units (Jane and Bill). And this time this makes sense to you - two staff should complete this task in half the time, so Jane and Bill should complete this task in one hour.

In Project, some tasks do behave this way - for example two staff can clear some shelves in half the time as it takes one member of staff. However other tasks do not. For example if 3 staff are scheduled to attend a one day meeting and you subsequently ask three additional staff to attend the same meeting, the meeting duration should not, perhaps unfortunately, reduce by half - it remains at the original duration.

Microsoft Project has traditionally been used in the construction/building industry where generally speaking many tasks are EFFORT DRIVEN. So Project sets all tasks to be EFFORT DRIVEN by default. However Project is now used in many different project situations involving managing tasks, people, other resources and costs where tasks do not necessarily behave in this way. So if you are starting a brand new project you need to decide if you want tasks to be EFFORT DRIVEN or not, or a mixture.

If you want all task durations to stay the same even if you change assigned resources you can turn off Project's default EFFORT DRIVEN setting and instead set all tasks to be "Fixed Duration". This means that if you change the resources assigned to the task, its duration will stay the same. If you want to change the EFFORT DRIVEN settings for all tasks in a particular Project, before the tasks are entered, ensure the application is open, then choose Tools, Options and select the Schedule tab.

Alternatively if you leave Project's default settings alone, so EFFORT DRIVEN is on, you can turn EFFORT DRIVEN off for individual tasks. This is done by double clicking a task and in the task information panel choose the Advanced tab.

The key to understanding EFFORT DRIVEN is to bear in mind what happens to the task duration if you change the assigned resource after the first resource has been assigned.

So hopefully you'll no longer be surprised if Microsoft Project changes a task duration automatically - it's because the task is EFFORT DRIVEN. Consider building up further skills in using Project by attending an instructor lead training course. The best ones are hands on and very instructive.