"we discovered that at minimum 10% of people on death row were innocent" How did we do that? I never heard this: certainly 10% of people on death row weren't e

"we discovered that at minimum 10% of people on death row were innocent" How did we do that? I never heard this: certainly 10% of people on death row weren't exonerated by DNA? This is some kind of shaky extrapolation I assume?