How to assess Yamato's armour quality

Was Yamato's thick armour actually low quality?

The evidence supports caution: it may have been lower quality than the best British or American armour, but that is not the same as being poor or ineffective.

Comparative quality should not be confused with absolute failure. The wrecks have not yielded enough testing to justify dismissing Japanese metallurgical competence. Japan had long experience with metallurgy and concentrated extraordinary resources on Yamato's armour and guns to offset American numerical superiority. British and American plate may still have been better because those industries accumulated extensive production and test experience. Calling Japanese plate lower quality is more defensible than calling it low quality without physical proof.