So what does the project management industry say about "quality"? Here are some samples.
PMBOK Guide V.5: "Quality - The degree to which a set of inherent characteristics fulfills requirements." This definition is followed by ten entries where quality is used as a qualifying adjective. In the body of the document, quality is treated as one of the Knowledge Areas under the heading of "Project Quality Management". However the Introduction states in part that "Managing a project typically includes, but is not limited to: Balancing the competing project constraints, which include Scope, Quality [etc., and] The relationship among these factors is such that if one factor changes, at least one other factor is likely to be affected. For example, if the schedule is shortened, often the budget needs to be increased to add additional resources to complete the same amount of work in less time. If a budget increase is not possible, the scope or targeted quality may be reduced to deliver the project's end result in less time with the same budget amount." Clearly, the PMBOK takes the position that scope and quality are two separate variable inputs.
Interestingly, the PMBOK states that: "Quality and grade are not the same concepts. Quality as a delivered performance or result is 'the degree to which a set of inherent characteristics fulfill requirements (ISO 9000)'. Grade as a design intent is a category assigned to deliverables having the same functional use but different technical characteristics. The project manager and the project management team are responsible for managing tradeoffs associated with delivering the required levels of both quality and grade. While a quality level that fails to meet quality requirements is always a problem, a low grade of quality may not be a problem."
Incidentally, "Grade" (presumably of quality) is defined in the Glossary as "A category or rank used to distinguish items that have the same functional use (e.g., 'hammer') but do not share the same requirements for quality 'e.g., different hammers may need to withstand different amounts of force).
If you can get your mind around all of that, you are doing well! Nevertheless, it is clear that PMBOK treats "Quality", whether of "inherent characteristics" or of "grade" as something that can be varied separately from "scope", where "scope" is defined as "The sum of the products, services, and results to be provided as a project."
International Standard ISO 21500 Guidance on project management takes a similar position. It does not discuss quality as a separate knowledge area, but it does recognize "quality" as a separate "constraint" along with scope, schedule, resources and cost. "Quality" is recognized as a subject group that includes "the processes required to plan and establish quality assurance and control. In fact it is one of several "baselines for carrying out the project" that may be subject to change. Just like the PMBOK, there are three project management process groups, namely Plan Quality, Perform quality assurance, and Perform quality control. The purpose of Plan quality is to determine the quality requirements and standards that will be applicable to the project, the deliverables of the project and how the requirements will be met based on the project objectives. This wording is very similar to the PMBOK, but ISO 21500 does not mention, perhaps wisely, the subject of "grade".
7. A Guide to the Project Management Body of Knowledge (PMBOK® Guide) Fifth Edition, Project Management Institute, PA, 2013, p556.
8. Ibid, p227.
9. Ibid, p6.
10. Ibid, p228.
11. Ibid, p542.
12. Ibid, p562.
13. In the case of scope and quality as inputs, "constraint" refers to the limiting boundaries to be recognized in the project.
14. International Standard ISO 21500 Guidance on project management, ISO, Geneva, 2012, p8.
15. Ibid, p13.
16. Ibid, p15.
17. Ibid, p16.
18. Ibid, pages 26, 27 and 27 respectively.
19. Ibid, p16.