Which part of the ISA/IEC 62443 series describes a methodology to develop quantitative metrics?

ISA/IEC 62443-1-3 is specifically titled "System Security Conformance Metrics" and introduces a methodology to develop quantitative metrics for assessing how well a system conforms to the ISA/IEC 62443 requirements.

“Part 1-3 defines a set of metrics to quantitatively measure the conformance of systems and components to the ISA/IEC 62443 series. It supports repeatable assessment and benchmarking.”

— ISA/IEC 62443-1-3:2013, Scope and Clause 5

This allows organizations to establish measurable KPIs and drive continuous improvement in IACS cybersecurity programs.