California's dearth of death row exonerations is certainly not due to incomparably accurate trials. DNA revelations caused some states to upgrade evidentiary standards. Aside from making DNA testing itself a right, California failed to do this, based not on reason, but on police and prosecutors pressing governors to veto minimal commonsense bipartisan reforms.  Throughout, the state has been plagued by police and prosecutorial misconduct scandals especially in the very few counties that dish out the vast majority of death sentences.  Nor are those counties' crime laboratories trustworthy.  California also has an exceptional record of prisoner abuse, with which its own courts have persistently found no fault, despite recurrent federal admonishments.  In addition, California is an outlier for super-secretive and hopelessly ineffective procedures for complaints against judges and attorneys. 
It follows that two national studies conservatively estimating that about 4% of death row inmates are innocent surely apply at least as conservatively in California.  Assuming the 4% rate, California's current death row population of 748  includes about 27 innocent inmates (4%=30, minus the already exonerated 3). Moreover, if and after living on death row long enough for his first appeal to be heard, a Californian innocent has only about a 15% (0.61/4) chance of exoneration. Elsewhere, such an innocent has about a 61% (2.44/4) chance of exoneration. The 2002 San Jose Mercury News Study
These indictments of California's criminal justice system echo those of a 2002 study undertaken by the San Jose Mercury, Death sentence reversals cast doubt on system:
A [San Jose] Mercury News review of hundreds of cases found that a state that touts itself as a national model in resources and legal protections for death-penalty defendants has the same systemic problems that are fueling concerns about capital punishment nationwide. In cases involving the murder of children, police officers, college students and the elderly, appeal courts reviewing death sentences are repeatedly finding incompetent lawyers, prosecutorial misconduct and judicial errors. A key finding of the review is that the [California] Supreme Court, which has become one of the nation's most pro-death penalty high courts, applies a different judicial standard than federal courts. When assessing errors in trials, the state's justices consistently find them to be "harmless," rather than grounds for overturning a death sentence. The result of the differing standards is reversals at the much-later federal level, decades after the crime. . . California hasn't taken corrective actions that other states have. . . California's Supreme Court is in greater conflict with federal courts than any other state's. [It] reverses 10 percent of death sentences, one of the lowest rates in the country. But federal courts have reversed 62 percent of the sentences affirmed by the California court, the highest rate nationally. . . The largest number of reversals have been for judicial error. . . Prosecutors and police also have contributed to the problem of death sentence reversals. . . [S]ince 1997 [the California Supreme Court] has reversed seven of the 67 death sentences for which it has produced full rulings, or 10 percent. By comparison,  other state high courts reversed about 40 percent. Even in Texas, which leads the country in executions, state courts reversed 31 percent, triple California's rate. . . [T]he 9th Circuit is more willing to uphold death sentences when state courts are more aggressive in weeding out flaws. Consider the case of Arizona, where the high court reverses two out of every five sentences it reviews, four times California's rate. When Arizona affirms a death sentence, the 9th Circuit tends to agree, reversing 42 percent of them, in line with the national average. . . The [Ninth Circuit] has many conservatives among its current and former judges.
After 2002, matters worsened. In 2003, the CSC upheld a death sentence despite a finding that three key witnesses had lied.  The only dissenters, Chief Justice George and Justice Kennard, have retired.
In light of four death penalty reversals this summer, some suspect a sea change due to the appointments of Justices Cue'llar and Kruger. As yet I sense no change.  The new justices signed off on Masters, none of the reversals affirmed innocence-based claims, and Cue'llar was the author of the decision that unanimously approved Seumanu's careful examination excuse for delay.  The first three cases were unanimous, based on mismanagement of the jury. In the fourth case, a 4-3 majority in January upheld the exclusion of hearsay stating that the defendant, who had participated in a robbery, had played no part in an incidental killing, and had been shocked by it. The arrival of the new justices before that decision became final resulted in reconsideration. Ultimately, the conviction was upheld. Only the death sentence was vacated, on the ground that even such weak mitigating evidence must be allowed in a capital sentencing hearing.Also Discussed In California's Death Penalty
The first part of California's Death Penalty:People v. Masters v. The California Supreme Court's Carefulness Con provides further details and further discusses:
- the implications of Propositions 62 and 66 with respect to the problem of exonerating the innocent
- the lasting legacy of the famous Rose Bird election
- why death penalty litigation is especially prone to error
- lessons learned from DNA exonerations
- death penalty caselaw:
- from Furman's 1972 nixing of the death penalty, to Godfrey's 1980 finding of inadequate review in Georgia, to Judge Carney's 2014 order finding California's review process unconstitutional
- from Herrera's holding that decay of evidence outweighs even strong exonerating evidence, to Ayala's inviting unreasonable denials of already excessively narrowed federal habeas claims
- from Thompson's refusal to remedy a clerical error by the Ninth Circuit that prevented the hearing of a meritorious innocence-based en banc petition, to Muhammad's refusal to grant a stay of execution to allow time to file a first petition to the nation's high court
- Williams and Cooper as exemplifying carelessness in California's review of innocence-based claims
As Texas law professor San Millsap puts it, the ultimate resolution of Cooper's case is "a test of sorts. . . [W]hether the state ... passes this test will say more about its real values than it does about Kevin Cooper or the miserable creatures who commit horrible crimes."  Jarvis Masters' pending state habeas petition also presents an acid test of the integrity of California's death penalty administration, as will be shown in the second part of this article.
 Cooper v. Brown, 565 F.3d 581 (9th Cir. 2009).
 People v. Masters, 62 Cal.4th 1019 (2016).
 Johnson v. Mississippi, 486 U.S. 578, 584 (1988). See also: People v. Keenan 31 Cal.3d 425, 430 (1982), citing Gardner v. Florida 430 U.S. 349, 357 (1977); Ford v. Wainwright 477 U.S. 399, 414 (1986); Beck v. Alabama 447 U.S. 625 (1980).