A good design specification should be clear, consistent and exact. Reasonable tolerances should be included and should be non-restrictive to encourage competition. A design specification provides explicit information about the requirements for a product and how the product is to be assembled. Design specifications should list minimum requirements and detail test, sampling and inspection
methods that will be used to ensure compliance with the specification.
Design specifications may have to comply with industry standards. For example, many pharmaceutical products are governed by British Pharmacopoeia for the UK and United States Pharmacopoeia for the U.S. A design specification must include all necessary drawings, dimensions, terms, and definitions of non-standard terms, and the materials used must be described in a manner so that bidders will be able to logically discern cost, process of construction, delivery and implementation of the requirements. Design specifications should state the desired outcome. These specifications carry a high degree of risk for the buying agency as the buyer will be held responsible for design omissions and related errors in the specifications.
A performance specification describes a product or service that provides a general functional recital of performance characteristics required to achieve an end result or outcome desired. The specification should be capable of integrating with existing systems and be interchangeable with parts, services or other basic elements of the operation‘s or product‘s expected outcome. Performance specifications provide the specific outcome that is required but not the method to achieve that outcome.
Performance specifications contain output and outcome measures and for this reason are often referred to as outcome or output based specifications. Output measures provide for a series of outputs that deliver a desired outcome. Outcomes should be measurable and define benefits delivered so that corrective deductions may be applied, as applicable, if service levels are not achieved.