Matching fingerprints involves judgment, skill and training and is extraordinarily reliable when done properly, its proponents say.

Critics charge that fingerprinting is far from infallible and is prone to more error and bias in the criminal justice system than is ever acknowledged.

The fingerprints of Brandon Mayfield, an Oregon lawyer mistakenly linked to the March 11 train bombings in Madrid, may just have been very similar to those of an Algerian man who Spanish authorities ultimately determined is a true suspect, said Simon A. Cole, a skeptic and author of "Suspect Identities: A History of Fingerprinting and Criminal Identification."

"Or while doing the analysis," he hypothesized, "someone was filled in on the idea that this guy was a Muslim lawyer with sympathies to Muslim groups and that might have biased them toward thinking there is a match."

Unlike DNA technology, said Cole, an assistant professor of criminology, law and society at the University of California at Irvine, no one has quantified the error rate in fingerprinting -- meaning there are no reliable estimates of how often the process implicates the wrong person.

Proponents say fingerprinting is extremely reliable. Joseph Polski, chief operations officer for the International Association for Identification, an organization for forensic scientists, said experts are highly accurate at comparing ridge endings, bifurcations and intervening ridges between two sets of fingerprints and determining whether they match.

Prints from crime scenes are converted into digital images and computers can be used to identify a range of possible matches. But only a human expert, Polski said, can make the final call.

Polski drew an analogy to an experienced pathologist, who can look at a single cell and spot cancer, while a new medical graduate might say only that something looks wrong.

Typically, "you don't find fingerprinting mistakes being revealed," said Roger Kahn, president of the American Society of Crime Laboratory Directors and the head of Ohio's crime labs. "Fingerprinting is extraordinarily reliable technology."

Cole said courts should demand that fingerprinting experts study the error rate in their technique and reveal it.