Boston bomber Dzhokhar Tsarnaev has been sentenced to death by a federal jury in the US for his part in the attacks on the 2013 marathon. But only a small proportion of those on Death Row are actually executed.

Between 1973 and the end of 2013, 8,466 people were sentenced to death in the United States, and 1,359 - about one in six - were executed.

"It's a death penalty in name only," says Frank R Baumgartner, a political science professor at the University of North Carolina at Chapel Hill.

He has studied the fate of people on death row and discovered that as of 31 December 2013

2,979 remain on death row
392 had their sentences commuted
3,194 had their death sentences overturned

Of those who have had their sentences overturned, the Death Penalty Information Center, estimates that 152 have been exonerated.

One conclusion that can be drawn from these figures is that, as Bryan Stevenson, executive director of the Equal Justice Initiative, put it on BBC Radio 4 earlier this year: "For every nine people that we have executed in America, we have identified one innocent person on death row."

Some people, of course, have died on death row of natural causes or suicide.

The proportion of people executed varies from state to state. Eighteen out of the 50 states have banned the death penalty, and just this week the state of Nebraska legislature voted in favour of abolishing it.

Of the 32 states where the death penalty remains in force, Baumgartner points out that Virginia executes a higher proportion of those sentenced to death than any other - about 72%.

"They are very strict about limiting appeals to 12 months. If your appeals aren't filed within 12 months, your case will be considered to be final.

"That's the only state to have such rules, and the only state that has more than 50% of cases carried out to execution.

"There are many states where it is extremely rare."

California has executed about 1%.

"They simply don't carry out their executions," Baumgartner says.