Wage Law

What are typical opt-in rates for FLSA collective actions? We've heard the figure 23% quite a few times, but we can't find any empirical data to support it. A December 2007 article published by the American Association for Justice reported that

Attorneys have found that the opt-in rate is usually low—less than 30 percent of the affected pool of workers.

Another article published last year by the American Bar Association reported that

Anecdotally, practitioners generally estimate that five to twenty percent of eligible plaintiffs will opt-in to a traditional opt-in case, while nearly all of the eligible employees will remain in an opt-out suit. ... [but] the low opt-in rate is not true, however, when FLSA claims are broughton behalf of a unionized workforce. Mullins v. City of New York (U.S. District Court for the Southern District of New York, Civil Action No. 1:04-cv-2979) [90% rate]; Abbey v. United States (U.S. Court of Federal Claims, Civil Action No. 07-272C) [70%].

Another article published in 2006 by defense firm Nixon Peabody LLP claimed that

Typically, the opt-in rate for these collective actions is between 15 and 30 percent.

Does anyone know of a solid empirical study that quantified the opt-in rates for collective actions over any recent period?