US death row inmates are ending up exonerated nine times more often than any other prisoner convicted of murder, according to a new report made public Monday.

"The most important thing we know about false convictions is that they happen and on a regular basis... Most false convictions never see the light of the day," said the authors of the study, professors of law Samuel Gross and Michael Shaffer of the University of Michigan.

After three years of research, the two experts, aided by their students, came up with the names of 2,000 inmates exonerated from 1989 to 2012.

They were able to develop profiles of 885 cases in a national registry, which is constantly updated and now includes 891 names listed on their website, exonerationregistry.org.

A quarter of prisoners exonerated of murder -- 101 out of 409 -- had been sentenced to capital punishment, according to the report.

And nearly half of all rape or murder convicts who had been later found innocent -- 341 out of 721 -- were about to be executed or remain in prison for the rest of their lives, the study said.

Ten were found innocent after their death.

"Most innocent defendants with short sentences probably never try to clear their names," Gross said. "They serve their time and do what they can to put the past behind them.

"Death sentences produce exonerations at nine times the rate for all homicide convictions".