Obama's argument for education vouchers

From his remarks in nominating Sonia Sotomayor for Supreme Court of the United States:

When Sonia was 9, her father passed away, and her mother worked six days a week as a nurse to provide for Sonia and her brother — who's also here today, is a doctor, and a terrific success in his own right — but Sonia's mom bought the only set of encyclopedias in the neighborhood, sent her children to a Catholic school called Cardinal Spellman, out of the belief that with a good education here in America all things are possible.

With the support of family, friends and teachers, Sonia earned scholarships to Princeton, where she graduated at the top of her class, and Yale Law School, where she was an editor of the Yale Law Journal, stepping onto the path that led her here today.

Oh, but one person's life experience is anecdotal. What does evidence tell us?