1. QAI Global Institute, Certified Software Quality Analyst (CSQA) Body of Knowledge. The CSQA BoK distinguishes between objective (quantitative) and subjective (qualitative) metrics. It specifies that objective metrics are based on direct, numerical counts (e.g., number of test cases, defect counts) and are intended to be free of the assessor's judgment. Subjective metrics, in contrast, are based on judgment and opinion (e.g., user satisfaction surveys). (Reference: CSQA Body of Knowledge, Skill Category 4: Metrics and Measurement).
2. Fenton, N. E., & Bieman, J. M. (2014). Software Metrics: A Rigorous and Practical Approach, 3rd Edition. CRC Press. As a standard university and academic text on the subject, this book defines measurement theory as it applies to software engineering. It clarifies that "A measure is objective if it returns the same value for the same object or event regardless of who is doing the measuring. For example, 'lines of code' is an objective measure... In contrast, a measure is subjective if it depends on the person doing the measuring. For example, 'understandability' is a subjective measure." (Chapter 2: The Basics of Measurement).
3. IEEE. (1998). IEEE Std 1061-1998 - IEEE Standard for a Software Quality Metrics Methodology. This standard provides a methodology for establishing quality requirements and identifying, implementing, analyzing, and validating software quality metrics. In Section 3, "Definitions," the standard implicitly supports the distinction by defining metrics based on verifiable, calculated values, contrasting with qualitative assessments that involve interpretation or judgment. The goal of the methodology is to create repeatable and valid metrics, which requires objectivity.