Again, attempts to prove correlation to wage increases were inconclusive. Productivity gains in the short run are viewed as being independent of wages. In either case, there appear to be to many variables to contend with. As he notes however, due to the interaction between the two, no casual interpretation of this trend can be made. The interaction that Sylos-Labini was addressing was that of the transfer of costs onto prices.
Longer tenure means a more experienced workforce.
Union emphasis on seniority gives experienced workers an incentive to train new workers. Statistical analysis may not allow for higher quality from a union work force. Management is forced to demand more of union workers. An increase in the supervisor to worker ratio. Better monitoring of worker performance.