Key decisions are made based on particular assumptions, for example, the existence or prediction of particular exchange rates.
If these items change then any plans based upon them may be invalid.
These could have a negative effect on the project.
They are often not under the control of the Project Manager and tend to be imposed.
For example, the personnel department may have an embargo on recruitment.
Whilst the project plan will have been prepared with these in mind, these constraints may still prove to be the undoing of the project.
Budget limits are another.
In the example below there has been some modification of the task durations and in the dependencies.
Note that, in this example, the total resource is still the same as it is assumed that the available resource is identical from all areas.
In addition a simple look at the project risks may lead to modification of task timing and resource or extra spend on equipment.
Whatever the reasons for modifying the schedule they should be recorded as part of the project justification.
Once this is in place it just remains to add resource costs to complete the picture.