It’s important to distinguish between "output certainty" and "outcome certainty". They are similar concepts, but not the same.

Outcome-based models refer to achieving a certain result (e.g. certification of a release meeting agreed upon criteria; Cost of Quality reduced by 25%).

Contrast that with Output-based constructs which refer to the completion of a given set of individual activities (e.g. run 10 test cases), typically completed for a set price and with some level of service level guarantees. This is another important performance-based style of contracting and is appropriate in many situations. Outputs are also the building blocks of a outcome-based model.