In the lab
Analysts prepare samples for DNA testing in the Virginia Dept. of Forensic Science's Northern Virginia Lab in Manassas, Va.
How accurate is forensic analysis?
Many forensic techniques developed in crime labs to aid investigators, and research into their limits or scientific validity was never a priority. Except for DNA, no method has been shown to be able to consistently and accurately link a piece of evidence to an individual or single source.
Learn more about the reliability of each type of forensic analysis.
DNA
DNA segments from a sample are compared to DNA profiles collected from convicted felons, crime scene investigations and unidentified individuals. Scientists calculate the probablility that two DNA profiles are from different people.
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Reliability weak points
Errors can occur if DNA samples are damaged or contaminated from improper handling. Limited amounts or mixtures of DNA profiles can increase misinterpretation of results.
Damaged
DNA sample
One case study
Houston Police Department Crime Laboratory
Houston shut its police crime lab's DNA division for several years after 2002 because of problems with the education and training of examiners, misleading testimony and improper evidence storage, leading to at least three exonerations and retesting of thousands of cases.
Fingerprint
A crime scene print is compared to a suspect's print or to those in a database. Analysts compare factors such as ridge count, shape, thickness, creases or scars and determine matching feature "points."
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In the lab
Fingerprints are analyzed in the latent prints section of the lab.
Reliability weak points
Matching and interpreting prints can be subjective and vary between examiners, whose level of training can range from formal programs to informal monitoring.
Matching
"points"
One case study
Houston Police Department Crime Laboratory
In 2009, a Houston police crime lab audit found irregularities in more than half of fingerprint examinations sampled. Officials hired consultants to review 4,300 cases and work through a 6,000 case backlog.
Handwriting
Professionals declared erroneous handwriting matches or genuine signatures in 6.5 percent and 3.4 percent of cases, respectively, in recent studies.
Reliability weak points
Handwriting from the same individual can be naturally inconsistent, so determining whether a comparison represents two different people can be unreliable.
Writing sample #1
Writing sample #2
Findings
Professionals declared erroneous handwriting matches or genuine signatures in 6.5 percent and 3.4 percent of cases, respectively, in recent studies.
Polygraph
Polygraph equipment measures the variability of a person's heart rate, blood pressure and respiratory rate when asked a series of questions.
Reliability weak points
Body changes registered
by polygraph equipment
can be subjective to interpret, caused by
anxiety rather than guilt.
Heart rate fluctuations registered on a polygraph may indicate nervousness, not guilt.
Findings
A 2003 National Academy of Sciences panel found polygraph testing lacks sufficient scientific validity and accuracy to justify its use in screening federal employees but useful as an investigative tool. Several federal circuit and state courts deem polygraph evidence inadmissible.
Firearm evidence
Bullets and shell casings are measured and examined for striation marks transferred from a gun barrel or for other impressions. Marks are compared to data collected from crime scenes or test firings.
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In the lab
The firearms and toolmarks section of the lab features a firearms reference library with hundreds of reference guns, a firing area, and pattern-based comparison.
Reliability weak points
Marks or "striations" on bullets are not necessarily unique to a specific firearm, and visually matching them can be subjective.
One case study
Detroit Police Department Crime Laboratory
Detroit closed its police crime lab in 2008 after errors were found in 10 percent of 200 criminal cases, prompting several re-trials.
Hair and fiber
Hair sample characteristics such as color, shaft thickness and length are compared to hairs from a known source. Fibers from clothing, carpet, rope, etc., are analyzed to determine type of material and environmental exposure.
Reliability weak points
Hair and fiber cannot be identified to one source. Hair comparisons must be confirmed by mitochondrial DNA analysis.
One case study
Santa Clara County Crime Laboratory
The Santa Clara County, Calif., district attorney replaced his crime lab chief after a murder case was dropped in 2007 and a wrongful armed robbery conviction was overturned in 2003 because of errors by a laboratory fiber expert.
Pattern and impression
Characteristics of a tire impression are compared to the actual tire suspected of leaving the track. Bite marks are compared to dental casts of a suspected individual. The method can also be used to exclude people of interest.
Reliability weak points
Visually matching impressions to sources can be subjective and varies between examiners. Skin may not accurately or consistently register bites.
Impression samples
degrade quickly.
One case study
North Carolina State Crime Laboratory
The North Carolina State Bureau of Investigation eliminated its bloodstain pattern analysis unit in 2011 after an examiner was videotaped celebrating after reproducing a result sought by a prosecutor.
Bullet lead composition
A bullet is analyzed to determine the ratio of lead or other chemicals it contains. Information is traced to a manufacturer, point of production, distribution or sale.
Reliability weak points
Tracing a bullet back to a specific box of ammunition is unreliable based on manufacturers' variations in packaging and frequency and range of distribution.
One case study
Federal Bureau of Investigations
The FBI abandoned bullet lead analysis in 2005 after the National Academy of Sciences called its work "unreliable and potentially misleading" and claims of linking a bullet to ones found in a suspect's gun or cartridge box misleading under federal rules of evidence.
SOURCE: National Research Council of the National Academies. VIDEO: Alexandra Garcia, Jonathan Forsythe GRAPHIC: Wilson Andrews, Jennifer Jenkins, Cristina Rivero and Spencer Hsu - The Washington Post. Published April 16, 2012.
Justice Department officials kept evidence flaws from defendants
They knew for years that sloppy forensic work at an FBI lab might have lead to wrongful convictions.
