The National Bureau of Economic Research (NBER) study contend that it has been difficult to explain how firms can find the right employees. Furthermore, t the study found that less work has been done to understand how different firms approach the hiring problem, what determines the firm-level heterogeneity in hiring strategies, and whether these patterns conform to theory. "...., the empirical relevance of the risk/incentive trade-off and the question of whether CEO pay arrangements reflect competitive markets and efficient contracting -- are likely to be very difficult to answer due to measurement problems." Why do you think the study made this assertion