The websites econjobmarket.org and AEAweb JOE are centralized job finding sites for economics PhDs. These have databases of application materials of thousands of job candidates, and the interviews many of them got. The subsequent jobs and publications of the job candidates are listed on the web. There are many empirical projects that can be done with this data, for example how certain keywords in recommendation letters predict the job that a candidate gets, or how the CV at the time of job application predicts future performance. One comparison that has been done in the sciences (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2572075/) is how recommendations of male and female candidates differ, i.e. what words are frequently used for one gender that are not used for the other. It is likely that economics recommendation letters contain similar biases.
The professors of top universities who have access to the databases of the job market websites have an advantage in hiring. They can predict which candidates perform well in the future and offer jobs to those. The employers without access to the databases are left with less promising candidates.