Wednesday, January 26, 2011

Proof Kinda Beyond a Reasonable Doubt

In a previous post, I described the concept of "Proof Beyond a Reasonable Doubt" as a convenient judicial myth. At the end of that post, I presented the following summary plot comparing the idealized concept against the reality of jury and judge verdicts.  I include that plot below for easy reference. Click to enlarge and clarify.
Judges and scholars, if forced to quantify the reasonable doubt threshold, tend place the threshold near 90%. While that number flies in the face of the plot above, it comports with quite a few studies attempting to quantify the threshold.

In his 1993 book Inside the Juror, Reid Hastie provides a summary of studies in which groups of people were merely asked to quantify reasonable doubt after being read a standard reasonable doubt instruction. I repeat his summary in a somewhat simplified form below.

In summary, when our country's jury pool is asked to quantify the reasonable doubt standard, they claim they set the threshold at 85%. That's not much lower than the threshold typically selected by judges and scholars. However even if jurors set the standard that high (the plot at the top of this post proves they don't), it seems to me still to be a problem.

If I have a ten-sided die with sides numbered from 1 to 10, I have a 90% confidence that any single roll will result in a number greater than 1. Do I therefore have proof beyond a reasonable doubt that I will not roll a 1? 

In this simple die rolling experiment, I know that if I set 90% as my threshold for reasonable doubt, I will be wrong nearly 10% of the time, over a large sample of tests. Is that acceptable?

If instead of rolling a die, I am sitting as a juror in a criminal trial, should I vote guilty if I am 90% confident in the guilt of the defendant? If I am willing to do so, does that mean there is a 10% chance the defendant is innocent?

If all my fellow jurors are willing to do so, to vote guilty when we are 90% confident, should we expect a wrongful conviction rate near 10%, just as Spencer and Allen have calculated we do?

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