The Evaluation of Forensic DNA Evidence (1996) / Chapter Skim
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3 Ensuring High Standards of Laboratory Performance
3 Ensuring High Standards of Laboratory Performance
Pages 75-88
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From page 75... ...
, it is important to understand that those estimates will be of little value if there has been an error in determining that the two DNA profiles match. A reported match in DNA samples that is the result of error in the handling or analysis of the samples could lead to the conviction of an innocent person, and an erroneously reported exclusion could also have serious consequences.
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From page 76... ...
· "Analysts successfully complete periodic proficiency tests and their equipment and procedures meet specified criteria. · "Reagents and equipment are properly maintained and monitored.
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From page 77... ...
requires extensive documentation of all aspects of laboratory operations (including the education, training, and experience of personnel; the specification and calibration of equipment and reagents; the validation and description of analytic methods, the definition of appropriate standards and controls, the procedures for handling samples, and the guidelines for interpreting and reporting data) , proficiency testing, internal and external audits of laboratory operations, and a plan to address deficiencies with corrective action and weigh their importance for laboratory competence.
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From page 78... ...
The guidelines cover all aspects of forensic analysis and affirm the key element of QA: the responsibility of laboratory managers for all aspects of laboratory operations and performance, including definition and documentation of standards for personnel training, procedures, equipment and facilities, and performance review. The DNA Identification Act of 1994 establishes a federal framework for setting national standards on QA and proficiency-testing.
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From page 79... ...
A second form of proficiency-testing, full-blind proficiency-testing, goes a step beyond open proficiency-testing in that the analyst does not know that a proficiency test is being conducted. It has been argued that full-blind testing provides a truer test of functional proficiency because the analysts will not take extra care in analyzing samples.
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From page 80... ...
There are circumstances in which undetected mishandling can lead to false matches; the genetic types of the samples might be determined correctly but the inferred connections among the samples can be incorrect because of sample mixup. Sample mishandling and incorrect recording of data can happen with any kind of physical evidence and are of great concern in all fields of forensic science.
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From page 81... ...
Safeguards against sample mishandling in the field include proper training of personnel involved in sample collection (such as crime-scene personnel) and submission of complete evidence items (rather than clippings or scrapings)
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From page 82... ...
Moreover, regular monitoring of test outcomes with standards and controls allows recognition of gradually emerging problems with reagents, equipment, controls, standards, and overall procedure that might otherwise be overlooked. For example, almost all North American forensic laboratories that perform VNTR analysis use DNA from the human cell line K562 as a positive typing control; correct sizing of restriction fragments from K562 DNA is prerequisite to accepting a typing result as reportable.
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From page 83... ...
It is generally found that the markers identified by the single locus probes used in forensic VNTR analysis and by PCR-based typing are detected in but a few nonprimate species; if such markers are used, that fact should obviously be taken into account. That is an advantage of DNA typing over enzyme and blood-group testing.
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From page 84... ...
Genetic-typing results, however, are usually unambiguous; one cannot make one genetic type look like another simply by wishing it so. In VNTR analysis, patterns must meet empirically defined objective match criteria to be said to match.
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From page 85... ...
SHOULD AN ERROR RATE BE INCLUDED IN CALCULATIONS? Some commentators have argued that the rate of profile matching due to laboratory error should be estimated and combined with the random-match probability (calculated with methods described in Chapter 4)
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From page 86... ...
· To estimate accurately, from proficiency test results, the overall rate at which a laboratory declares nonmatching samples to match, as has been suggested, would require a laboratory to undergo an unrealistically large number of proficiency trials. Suppose that two laboratories each have under specific conditions a false-positive error rate of 0.10% one match per 1,000 nonmatching proficiency trials.
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From page 87... ...
However, there is no need to debate differing estimates of false-match error rates when the question of a possible false match can be put to direct test, as discussed in the next section. RETESTING A wrongly accused person's best insurance against the possibility of being falsely incriminated is the opportunity to have the testing repeated.
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From page 88... ...
. Proficiency Tests Regular proficiency tests, both within a laboratory and by external examiners, are one of the best ways of ensuring high standards.
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