There are three important elements that you need for conducting an assessment:

• An Assessment Model
• An Assessment Method
• One or more Competent Assessors

ISO/IEC TR 15504 defines a set of requirements for an Assessment Model and an Assessment Method in the normative parts of the document set (parts 2 and 3 respectively). An assessment that meets these requirements is referred to as a 15504-conformant assessment.

ISO/IEC TR 15504 also defines a Reference Model for software processes and process capability that forms the basis for software process assessment. The Reference Model is applicable to any software organization wishing to establish and subsequently improve its capabilities in the acquisition, supply, development, operation, evolution and support of software. The model does not presume particular organizational structures, management philosophies, software life cycle models, software technologies, or development methodologies.

An actual software process assessment is conducted using an Assessment Model, not the Reference Model.

There can be more than one Assessment Model that satisfies this architecture and its requirements. In the ISO/IEC TR 15504 there is a complete exemplar Assessment Model that can be used during an assessment. Additional Assessment Models that are claimed to be conformant to the above architecture and the requirements defined by ISO/IEC TR 15504 are being provided by vendors and service providers in the marketplace.

ISO/IEC TR 15504 does not define an explicit assessment method. It does, however, define the requirements for an assessment method. This means that there may be many assessment methods available, all of which meet these requirements. There are a number of organisations that provide public or commercial assessment methods that are claimed to meet the method requirements defined by ISO/IEC TR 15504.

There are a few additional requirements for 15504-conformant assessments that are defined in the normative parts of ISO/IEC TR 15504. These requirements are intended to ensure that good assessment practices are followed, and also to ensure consistency across all assessments that are based on ISO/IEC TR 15504.