Aspiring bank robbers, take heed. Recent statistical analyses of confidential bank data suggest that mountains of riches aren't in your future but that a jail cell is.

“The return on an average bank robbery is, frankly, rubbish,” wrote the authors of a June article about the economics of British bank robberies in Significance, a bimonthly statistics magazine published by the American Statistical Association and the Royal Statistical Society. To complete their research, economists Neil Rickman and Robert Witt of the University of Surrey and Barry Reilly of the University of Sussex spent months negotiating with the British Bankers' Association to obtain confidential records detailing 364 bank heists that occurred in the U.K. between 2005 and 2008. Such detailed data are not available in the U.S., where, in contrast, if banks even record them, they are lost in the anonymity of the FBI's quarterly reports on bank robberies.

The statistics reveal that the average British bank robbery is committed by 1.6 thieves and nets $31,900 per heist, with a standard deviation of $84,000. Assuming an equal share, the average take was $19,900 per robber per heist—roughly equivalent to a coffee shop barista's annual salary.

Wielding a gun increased the average haul by $16,100, as did adding more accomplices to a raid. Going alone, however, netted the average robber more money because the amount gained by increasing a heist party did not outpace the hit taken by splitting an individual robber's spoils.

The sums are not chump change, Rickman notes, but bank robbing is risky business. Roughly 33 percent of British bank heists end with no robber earning anything, and about 20 percent of raids end in capture. The odds of arrest get worse as a robber attempts more heists. On a robber's fourth heist, for example, the odds of capture compound to 59 percent. “Somehow I expected most bank robbers to be doing much better than what the data actually show,” Rickman says.

Some would-be criminals do better than others. Economist Giovanni Mastrobuoni of the University of Turin's Carlo Alberto College in Italy takes issue with the article's lack of attention to the professionals, who presumably raked in most of the $11.6 million stolen from British banks between 2005 and 2008. The article suggests, for example, that fast-rising bulletproof screens in some banks reduce a robbery's success by one third. “But I'd argue that the dumb robbers target banks with fast screens, while professional ones scope out the place thoroughly before robbing,” Mastrobuoni says. Rickman counters that information on professionals is even harder to come by because it requires access to confidential police and bank records. The new report, economists say, underscores the need for more and better data on bank heists.