In searching Google Scholar I came across a paper on using Six Sigma to improve competitiveness in HR. https://poseidon01.ssrn.com/delivery.php?ID=317112116088089094090119119015107004053017063051087026113097071127097016102124073024053029061030029060000115124123112024004066053061042009079100028121125069022091058047048086000065082068080111019108070074026100089008117069097125080068118020107017081&EXT=pdf&INDEX=TRUE