The three services:
- Fixed Scope
- Fixed Cost
- Fixed Deadline
But you can pick only two
- Fixed scope and fixed cost won’t give you a fixed deadline
- Fixed cost and fixed Deadlines won’t give you fixed scope
- Fixed deadlines and fixed scope won’t give you fixed cost
Fixed Scope and Fixed Cost
Locking the scope early by pre-determining the entire product backlog while also agreeing on the number of teams will in nature make it impossible to have a fixed deadline. The teams are expected to learn more as they work on the product backlog and interface with the customers to bring real value.
Fixed cost and fixed deadlines
Locking the number of teams working on the product backlog while also having a fixed deadline will make it impossible to promise to deliver all of the items in a pre-determined product backlog. You can always have a fixed deadline. In fact, you can have a fixed deadline after each sprint since each increment should be potentially releaseable.
Fixed deadlines and fixed scope
Locking the content of the product backlog while also giving a specific date where all items should be delivered will not be possible with a fixed number of teams. Teams are expected to learn more about the details as they go along and content may shrink or grow and as such need more teams to reach the deadline.
In the ideal agile world, the only thing you should fix early is the cost, i.e. number of teams. The teams are expected to be stable over a longer period to increase learning in the team and improve collaboration internally and externally.