The most critical ingredient of a successful procurement of services is the development and documentation of the requirements – the statement of work. The S.O.W. identifies what the contractor is to accomplish. It first clearly identifies the primary objective and then the subordinate objectives. One of the goals of the S.O.W. is to gain understanding and agreement with a contractor about the specific nature of the technical activity to be performed. The S.O.W. also impacts the administration of the contract by defining the scope of the contract, that is, what the contractor is supposed to do and the purchaser supposed to receive.
Source: Dobler, D.W., & Burt, D.N. (1996). Purchasing and Supply Management. (6th ed.). New York: McGraw Hill.