Note that the next lowest exoneration rate is 1.29%, for Alabama--four times California's rate. States with under 200 death sentences are less significant, but only two are arguably comparable: Nevada (1 of 151=0.66%) and Arkansas (0 of 122=0%). Perhaps surprisingly, Louisiana has one of the highest exoneration rates (10 of 166 = 6.02%), twenty times California's rate.
 There are (at time of writing) 462 "No Action" rows in the state's list of condemned inmates. Counting only those sentenced to death whose appeals have been decided, California's exoneration rate is 3 in 489 (951-462) = 0.61%. The exoneration rates given for other states also include significant counts of as-yet-undecided appeals. I super-conservatively estimate that excluding these cases would raise the extrinsic exoneration rate from 2.17% to only 2.44% (in California it almost doubled), to obtain my quarter estimate (0.61% vs. 2.44%). Realistically, I guess the true factor is about one fifth. To calculate the exact ratio requires deducting the as-yet-undecided appeal counts (which I do not know) from each state's death sentence totals.
 A Broken System: Error Rates in Capital Cases, 1973-1995 (2000), J. Liebman et al. (at i; boldface in orig.):
Capital trials produce so many mistakes that it takes three judicial inspections to catch them--leaving grave doubt whether we do catch them all. After state courts threw out 47% of death sentences due to serious flaws, a later federal review found "serious error"--error undermining the reliability of the outcome--in 40% of the remaining sentences.
 See Schwartzenegger Vetoes Justice, Washington Post, Nov. 5, 2007 (three simple inexpensive bills vetoed); Jerry Brown vetoes bill aimed at holding prosecutors [very, very slightly] more accountable, Washington Post, Oct. 1, 2014.
 See Too
Broken To Fix (Part 1), by Harvard's Fair Punishment Project. Nationwide, only 16 (of 3,143) counties have issued
five plus death sentences from 2010-2015. Five of them--Kern, Los Angeles,
Orange, Riverside, and San Bernadino counties--are in south California, where
there have been successive scandals involving the use of snitches to fabricate
evidence. Three key features in such counties are
Prosecutors," "Inadequate Defense," and "Racial Bias." In particular:
[I]n America, a tiny handful of prosecutors account for a wildly disproportionate number of death sentences. Indeed, just three prosecutors personally obtained a combined 131 death sentences, the equivalent of one in every 25 people on death row in America today. Those same prosecutors amassed findings of misconduct in 33 percent, 37 percent, and 46 percent of their cases, respectively. . . [Death row e]xonerations are common in jurisdictions with overly aggressive prosecutors and inadequate defenders.
To the last sentence I would add "except in California, where death row exonerations are all but non-existent."