An SOW is the portion of a Government contract which establishes and defines all non-specification requirements for contractors' efforts either directly or with the use of specific cited documents. A PWS on the other hand is a statement of work for performance-based acquisitions that describes the required results in clear, specific and objective terms with measurable outcomes. We'll explain these terms further.
Statement of Work (SOW)
An SOW specifies the work to be done in developing or producing the goods to be delivered or services to be performed by a contractor. According to DoD, the preparation of an effective SOW requires both an understanding of the goods or services that are needed to satisfy a particular requirement and an ability to define what is required in specific, performance-based, quantitative terms. A SOW prepared in explicit terms enables offerors to clearly understand the government's needs. This facilitates the preparation of responsive proposals and delivery of the required goods or services.
After contractor selection and contract award, the contract SOW becomes the standard for measuring contractor performance. As the contracted effort progresses, the government and the contractor will refer to the SOW to determine their respective rights and obligations. In this respect, the SOW defines the contract and is subject to the interpretations of contract law. In a dispute concerning performance, rights, or obligations, clarity in defining requirements is of utmost importance.
Performance Work Statement (PWS)
The PWS states requirements in general terms of what (result) is to be done, rather than how (method) it is done. The PWS gives the contractor maximum flexibility to devise the best method to accomplish the required result. The PWS must be written to ensure that all offerors compete equally. The Government must remove any features that could restrict a potential offeror. However, the PWS must also be descriptive and specific enough to protect the interests of the Government and to promote competition. The clarity and explicitness of the requirements in the PWS will invariably enhance the quality of the proposals submitted. A definitive PWS is likely to produce definitive proposals, thus reducing the time needed for proposal evaluation.
When a contract is awarded, the PWS is legally binding upon the contract and the Government. It provides an objective measure so that both the contractor and the Government will know when increments of work are completed and payments are justified.
The Government issues a solicitation for grass cutting. Under a SOW, the requirements might specify the contract cut the grass every two weeks. The contractor has to mow the grass every two weeks, whether it grew or not. Under a PWS, the requirements might specify that the grass length be maintained at a height of between one and two inches. Its up to the contractor how to meet that requirement.