A Quick Research on False-Positives
False-positives-It is a well-established psychological phenomenon that people tend to see what they expect to see, particularly in ambiguous situations. This is a quick research done on false-positives and how they can effect the integrity of our justice system.
A Bryn Mawr student gets caught at an airport with condoms filled with flour. She and her dorm mates made them as gag squeeze-toy stress relievers during finals. A field test -- conducted twice -- indicates that the condoms are filled with opium, cocaine, and amphetamines. The girl spends the next three weeks in jail on drug charges that could bring 20 years in prison.A jail guard recognizes the girl from volunteer work, believes her story, and tips off local Catholic groups to her cause. The group gets her competent representation, who demands the substance be retested. More extensive tests show it was flour after all.
There is an article which references cases involving drug sniffing dogs in the field giving false positives 8 percent of the time or more and studies where in "artificial testing situations" they gave false positives between 12 and a mindboggling 60 percent of the time. Generally dogs are brought in when there is already probable cause, but should the police begin to feel they don't need that reason, we can expect the proportion of false alerts to rise. Baye's Rule- in short a highly accurate test will give overwhelming numbers of false positives if the size of the population tested is far larger than the size of the possible true positives.
DNA- Last year it was discovered that 26-year-old Lazaro Soto Lusson was mistakenly charged with multiple felonies because the Las Vegas police crime lab switched the labels on two DNA samples. While in jail on an immigration hold, Lusson's cellmate, Joseph Coppola, accused him of rape. Police took DNA samples from both men to investigate the allegation. While undergoing the analysis, they ran the samples against the state database and matched Lusson's mislabled DNA to two unsolved sexual assaults. Lusson faced life in jail and was incarcerated for over a year before this mistake was discovered. Similar sample switch errors have led to false incrimination in rape cases in Philadelphia and San Diego.
Misinterpretation of DNA tests led to the false conviction of Timothy Durham in Tulsa, Oklahoma. Durham was convicted of raping an eleven-year-old girl and sentenced to 3,000 years in prison, despite having produced 11 alibi witnesses who placed him in another state at the time of the crime. The prosecution's case rested almost entirely on a DNA test, which showed that Durham's genotype matched that of the semen donor. Post-conviction DNA testing showed that Durham should have been excluded as a possible suspect, and re-analysis of the initial test showed that the misinterpretation arose from the difficulty of separating mixed samples. The lab had failed to separate completely the male and female DNA from the semen stain, and the combination of alleles from the two sources produced a genotype that could have included Durham's. Durham was released from prison in 1997 after serving 4 years in prison.
REFERENCES:
http://www.aclu.org/privacy/genetic/14995pub20031106.html
http://www.reason.com/hitandrun/2005/02/belated_caballe_1.shtml
http://www.theagitator.com/archives/2005_12.php
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A Bryn Mawr student gets caught at an airport with condoms filled with flour. She and her dorm mates made them as gag squeeze-toy stress relievers during finals. A field test -- conducted twice -- indicates that the condoms are filled with opium, cocaine, and amphetamines. The girl spends the next three weeks in jail on drug charges that could bring 20 years in prison.A jail guard recognizes the girl from volunteer work, believes her story, and tips off local Catholic groups to her cause. The group gets her competent representation, who demands the substance be retested. More extensive tests show it was flour after all.
There is an article which references cases involving drug sniffing dogs in the field giving false positives 8 percent of the time or more and studies where in "artificial testing situations" they gave false positives between 12 and a mindboggling 60 percent of the time. Generally dogs are brought in when there is already probable cause, but should the police begin to feel they don't need that reason, we can expect the proportion of false alerts to rise. Baye's Rule- in short a highly accurate test will give overwhelming numbers of false positives if the size of the population tested is far larger than the size of the possible true positives.
DNA- Last year it was discovered that 26-year-old Lazaro Soto Lusson was mistakenly charged with multiple felonies because the Las Vegas police crime lab switched the labels on two DNA samples. While in jail on an immigration hold, Lusson's cellmate, Joseph Coppola, accused him of rape. Police took DNA samples from both men to investigate the allegation. While undergoing the analysis, they ran the samples against the state database and matched Lusson's mislabled DNA to two unsolved sexual assaults. Lusson faced life in jail and was incarcerated for over a year before this mistake was discovered. Similar sample switch errors have led to false incrimination in rape cases in Philadelphia and San Diego.
Misinterpretation of DNA tests led to the false conviction of Timothy Durham in Tulsa, Oklahoma. Durham was convicted of raping an eleven-year-old girl and sentenced to 3,000 years in prison, despite having produced 11 alibi witnesses who placed him in another state at the time of the crime. The prosecution's case rested almost entirely on a DNA test, which showed that Durham's genotype matched that of the semen donor. Post-conviction DNA testing showed that Durham should have been excluded as a possible suspect, and re-analysis of the initial test showed that the misinterpretation arose from the difficulty of separating mixed samples. The lab had failed to separate completely the male and female DNA from the semen stain, and the combination of alleles from the two sources produced a genotype that could have included Durham's. Durham was released from prison in 1997 after serving 4 years in prison.
REFERENCES:
http://www.aclu.org/privacy/genetic/14995pub20031106.html
http://www.reason.com/hitandrun/2005/02/belated_caballe_1.shtml
http://www.theagitator.com/archives/2005_12.php
HOME
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