Last edited 19 May 2020

Input specification vs output specification

Traditional specifications are ‘input specifications’. They are prescriptive, that is, they describe in detail the products, materials and work (the inputs) required by a contract. Typically, the development of the solution required by the contract is complete and no (or little) further development is required by the supplier. The specification is considered to be a ‘closed specification’.

Prescriptive input specifications give the client certainty about the end product when making investment decisions, but place a greater burden on them to ensure the product has been thoroughly considered, developed and described. They tend to be used where a very specific end result required.

Output specifications define only the outcomes (or outputs) that are required by the contract, that is, what it will enable the client to do. They are open specifications that do not attempt to address how those outputs might be achieved. For example, an output specification might require the provision of a space for teaching 30 primary pupils, but it would not specify the nature of the space, the types of doors and windows etc. The standard for what is required might be defined, for example, by reference to performance targets.

It is argued that output specifications can get the best value from the supply team by allowing them to adopt innovative solutions to the client's requirements and reduce whole-life costs rather than simply developing the client's pre-conceived solutions at the lowest price.