PMI and most other project management educators use the notion of the Triple Constraint - Cost, Schedule, and Scope. This is sometimes called the Iron Triangle. It is call Iron because when you change one thing, the other two change as well. This has been talked about here before.
Cost we know about. Cost starts with the Budget and then includes the Actual Cost of performing the work - consuming that budget. Schedule is the same. We have a Plan and we execute the project against that Plan. As the project proceeds, we are either on plan or off plan. That is we are ahead or behind schedule.
Scope is supposed to be a description of the deliverables from the project. The Outcomes of the projects schedule and budget. But in the PMI vernacular, scope fails to explicitly include several things.
- Quality - does the product work as defined?
- Technical Performance - does the product work as needed?
- Key Performance Parameters - does the product meet the needs of the consumer?
These are all wrapped into the word scope. Here's a better way to look at the Triple Constraint.
- Technical Performance Measures
- Measures of Effectiveness
- Measures of Performance
- Key Performance Parameters