A St. Louis County grand jury on Monday decided not to indict Ferguson, Missouri, police officer Darren Wilson inthe August killing of teenager Michael Brown. The decision wasn’t a surprise — leaks from the grand jury had ledmost observers to conclude an indictment was unlikely — but it was unusual. Grand juries nearly always decide toindict.Or at least, they nearly always do so in cases that don’t involve police officers.Former New York state Chief Judge Sol Wachtler famously remarked that a prosecutor could persuade a grand juryto “indict a ham sandwich.” The data suggests he was barely exaggerating: According to the Bureau of JusticeStatistics, U.S. attorneys prosecuted 162,000 federal cases in 2010, the most recent year for which we have data.Grand juries declined to return an indictment in 11 of them.Wilson’s case was heard in state court, not federal, so the numbers aren’t directly comparable. Unlike in federalcourt, most states, including Missouri, allow prosecutors to bring charges via a preliminary hearing in front of ajudge instead of through a grand jury indictment. That means many routine cases never go before a grand jury.