Monday, July 26, 2010

On The Rate of Wrongful Conviction: Chapter 1.3

As I have mentioned five times previously, I am preparing a monograph on the rate of wrongful conviction. Each chapter will deal with one estimate of that rate, beginning with zero and ending beyond 10%. I am posting the draft chapters here, as I write them.  I have so far posted the following: 

Chapter 0.027: The Scalia Number
Chapter 0.5: The Huff Number
Chapter 0.8: The Prosecutor Number
Chapter 1.0: The Rosenbaum Number
Chapter 1.4: The Poveda Number

I work slightly backwards once again here, filling in with the Police Number at 1.3 percent. It's a short post.

Chapter 1.3
The Police Number

The Ramsey Ohio survey study introduced in Chapter 0.8 was so comprehensive that it allows me to determine four separate wrongful conviction estimates: one each for prosecutors, police, judges, and defense attorneys. The prosecutors were the most conservative in their estimate, guessing that the wrongful conviction rate for the country is 0.8%.  That number represents the median value from the survey, the value that split the prosecutors into two evenly-divided groups: one that guessed higher and one that guessed lower.

The second most conservative group within the Ramsey survey was the police. Regarding the survey question at hand, Ramsey received responses from 267 chiefs of police and county sheriffs. Recall that Huff had only 353 respondents from all four groups.

The police responses from the Ramsey survey are categorized below.

I will define the Police Number for the wrongful conviction rate in the same manner I defined the Prosecutor Number: I’ll determine the median police guess at the wrongful conviction rate using a plot. That plot is presented below. Based on that chart, I claim the single best number to represent the wrongful conviction rate estimated by the Ohio police responding to the Ramsey survey is 1.3%.

Consider the significance of the Police Number. If it is applicable to all people convicted, not just those who go to trial, it suggests that even the police believe we have 32,500 people wrongfully incarcerated in this country today.


As mentioned in the earlier chapter dealing with the Ramsey survey, I noted that Ramsey was clever enough to ask for separate guesses of the wrongful conviction rate: one for the country as a whole and one for the respondent’s jurisdiction. It’s interesting to compare the two.

For the police, the national median guess was 1.3%, as just reported. For that same group of police, the median guess for their jurisdiction was 0.2%. On average, the police believed the wrongful conviction rate in their jurisdiction was less than one-sixth that of the national average. This bias is one of the reasons the Huff number is so low. Huff failed to clearly discriminate between guesses for the nation as a whole and the respondent's jurisdiction.

The number for prosecutors, by comparison, are 0.8% and 0.2%.

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