No, and it is not a substitute for one. Accreditation measures you against published standards and asks whether you meet them. An assessment asks whether the institution is working, which is a different question with a different answer.

For schools, they pair well. Operational Vitality is one of the four NSBECS domains, so an assessment produces evidence you will need — and it usually surfaces things a self-study is not designed to catch, because a self-study is written by the people being evaluated.

Parishes have no equivalent cycle, which is precisely why an outside read matters more there, not less. Most parishes have never had one.