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The Birch (Employee Growth) Index is an economic indicator of employment. It multiplies absolute job growth by relative job growth, which reveals the employment-creation power of differently sized enterprises.
It was devised by economist David L. Birch at the Massachusetts Institute of Technology. In 1980 Birch published an influential study that showed 70% of job growth in the U.S. came from enterprises with fewer than 100 employees.
References
The Positive sum strategy: harnessing technology for economic growth, by Ralph Landau, Nathan Rosenberg, National Academy of Engineering, page 519