An Outcome Based Specification (OBS) focuses on the desired outcome of a service in business terms, rather than a detailed technical specification of how the service is to be provided; this allows providers scope to propose innovative solutions that might not have occurred to the procurement team. Outcome should be distinguished from output, which is the measurable results of a set of inputs. The example of difference between outcome and output is written at the bottom of page 123 in the study guide.
Outcomes should be the starting point in making new specification. However, using outcome-based specification has some setbacks:
- First, it is not easy to measure the outcomes. Usually, outcome of a project is a statement like 'increase customer satisfaction', 'maintain ambient temperature' or 'provide a convenient way to do something'. They are not easy to measure as output.
- Second, sometimes the desired outcomes require time to be materialised
- Third, outcomes can be ambiguous

The buying organisation is keen to encourage innovation so they should use the outcome or output based specification. In an outcome-based specification, umbrella statements like 'good quality', 'ambient temperature', 'convenient way' are often used. This may confuse the suppliers, and it's hard to check the solution that supplier offers. On the other hand, ouput-based specifications often include measurable requirements. For example, a specification for air conditioning system states that the system should maintain the room temperature at 19-24 degrees Celsius. Therefore, output specification is more appropriate in this case.