It is possible that some of these cases might involve DNA from different people with remarkably similar genetic makeup. But in a number of cases, the slight differences have already been traced back to errors in the profiles stored in DNA databases: What appeared to be DNA profiles of different people was, in fact, the profile of the same person’s DNA, according to documents.

A review by the New York City medical examiner’s office determined that six of the 166 cases were attributable to errors in DNA profiles it generated from crime scenes in New York, according to a letter the office sent to the state panel.

Since the discovery of the 166 errors at the national level, the New York State Police has also changed the search parameters used to comb DNA profiles in the state database, finding additional errors. The state database includes profiles that do not meet the F.B.I.’s standards for inclusion in the federal database because they come from DNA that is too degraded or that came from complex mixtures of DNA at crime scenes.

In two of the errors discovered in the state database, analysts at the city’s medical examiner’s office made mistakes as they tried to discern a DNA profile from raw data, which appears graphically as a series of peaks, somewhat like the record a seismograph produces.

“These revelations spotlight how human error can detract from the reliability of the testing process,” said Alan Gardner, the head of Legal Aid’s DNA unit, which is challenging the city medical examiner’s methods for discerning DNA profiles in complex mixture cases.

In court, prosecutors often describe the strength of DNA evidence against a defendant with numbers that can run into the billions — expressing how unlikely it is that a person chosen at random would also have a DNA profile linked to the crime scene. But the rate of errors by a lab or a technician, a less dramatic topic, can be a much more relevant statistic, many defense lawyers and some scientists said.

“If we say there is a 1-in-10-quadrillion chance that someone else might have the same DNA profile, but there is also a 1-in-10,000 chance that there was a mistake in generating the profile, the only number the jurors should be paying attention to is the error rating, said Dr. Krane, who was once on a forensic science commission for the State of Virginia and now consults with defense lawyers on DNA cases.