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Usability is similarly ill-defined and hard to measure.Human factors are very well studied and standardized, and there is a well-established discipline called "Human Factors Engineering", which also provides established test and evaluation methods. Human Factors research is considered solid and well-established because it has been built on rigorous experimental psychology and engineering principles developed over more than a century, with systematic methodology and empirical validation. Even if much of it is unknown or ignored by computer science or programming language design, there are many disciplines where Human Factors Engineering is critical (see e.g. ANSI/AAMI HE75).
Usability is therefore neither ill-defined nor hard to measure. Several ISO 9241 series standards address textual and command-based interaction directly relevant to programming language assessment. ETSI EG 202 116 provides extensive human factors guidelines for command language style. ITU-T Recommendation Z.361 provides guidelines for Human-Computer Interfaces in telecommunications management, incorporating the ISO 9241-10 dialogue principles. ISO/IEC TR 24772 addresses programming language vulnerabilities specifically from a human factors perspective.
E.g. Ada did have substantial human factors considerations documented in its design rationale, directly contradicting the notion that such principles don't apply to professional developers or programming languages. It's rather that computer science seems to continue ignoring established fields ("Not invented here"). Human factors in software development have been overlooked by researchers in the software engineering and development research areas, despite their obvious relevance. So what is lacking is primarily interest or willingness, not the fundamentals and means.