One of the key issues in a case argued Thursday before the California Supreme Court is how to describe the significance of a "cold hit," when a criminal suspect is first identified through a DNA database search.

The question has been the subject of scientific controversy for more than 15 years and centers on how to calculate the probability that such a match would be the result of coincidence.

Though people's DNA is unique, genetic profiles are not. They are based on a sliver of markers on human chromosomes. Different people can share markers and in some cases even unrelated people share the same genetic profile by chance.

When other evidence has led investigators to a suspect, prosecutors can legitimately argue that the chance of such a coincidental "match" between DNA from the crime scene evidence and the suspect is remote. They cite a calculation called the random match probability: the chance that a person, selected randomly in one try from the general population, would happen to have a profile that matched the crime scene evidence. (This is sometimes called the "rarity" statistic.)

But when a database is searched, hundreds of thousands, sometimes millions, of comparisons are made to profiles within a database, increasing the odds -- sometimes significantly -- that a coincidental match will occur.

The Dennis Louis Nelson case, which involves a 1976 murder in Sacramento in which the suspect was first identified by a database search, thrusts the California Supreme Court into this controversy, in which many leading scientists are at odds with the current practice in the courts.

The court's decision could dictate how statistics in cold hit cases are presented to juries in California -- something that probably would be influential across the country.

In considering the issue over the last 12 years, the two leading scientific bodies in the field -- the National Research Council and the FBI's DNA Advisory Board -- have reached the same conclusion: In cold hit cases, jurors should be given an adjusted calculation, called the database statistic, that adjusts for the number of comparisons made within a database.

This calculation takes into account the number of comparisons made during the search and presents jurors with increased odds of a coincidental match, the panels found.

Most of the scientists interviewed by The Times say they still favor using the database statistic -- some along with the rarity number -- but not all experts agree.

The scientific panels' recommendations are being followed by few if any prosecutors and crime labs across the country, The Times has found.

Among those who favor a different approach are a vocal group of dissenting statisticians who contend that no statistical adjustment is necessary. Among other arguments, they say that a criminal database search strengthens the significance of a DNA match rather than diluting it because the search rules out other known criminals.

Some experts object to that complex approach to calculating probabilities, which they say violates the legal principle of presuming innocence until guilt is proved. The approach is not used in American criminal cases, and both the National Research Council and DNA Advisory Board rejected it.

Prosecutors and crime lab analysts argue that the various statistical approaches are not at odds but provide different answers to different questions. It is for the courts -- not scientists -- to decide which of those questions is most relevant for jurors to hear, they say.

But most favor presenting the random match probability, which describes the rarity of a profile in the general population but does not account for the database search.

Many scientists believe this approach is misleading. "They're not telling the full truth," said Ranajit Chakraborty, a leading scientist in the field and frequent expert witness in DNA cases. "To that extent, they're wrong."

Many prosecutors and lab analysts argue that the database search has no effect on the odds of a match if the suspect's DNA profile is retested against the crime scene DNA. Scientists on all sides of the debate agree that this notion is wrong: Retests do nothing to change the odds of a coincidental match.

Defense attorneys, meanwhile, generally favor using the database statistic, though judges often don't allow them to present it in court.

Some have argued that until there is a scientific consensus on the question, the evidence in DNA cold hits should be kept out of courts.

In a friend of the court brief filed in the Nelson case, however, public defenders urged the court to adopt yet another approach, which is considered by some to be scientifically sound but unnecessarily conservative.

The California Supreme Court may consider all of these views in deciding whether a scientific consensus exists on the question and which statistics should be used.

The court's decision may hold more weight than two appellate rulings that many scientists say misconstrued the issues at play.

In People vs. Jenkins, a Washington, D.C., murder trial, the defense cited the dispute as evidence that a scientific consensus did not exist on what statistics should be presented. The trial court judge agreed and threw out the DNA evidence. On appeal, however, the DNA evidence was reinstated.

The D.C. high court found that the different camps in the dispute were merely providing accurate answers to different questions. Their ruling held that the random match probability is always relevant.

The issue also came up again in People vs. Johnson, a Sacramento cold hit murder case. On appeal, the court found that the database search is merely an investigative tool, and therefore need not be presented to a jury.

Many scientists and legal experts say the courts in both those cases misunderstood the statistical implications of a database search. The ruling in Nelson is expected this summer.

jason.felch@latimes.commaura.dolan@latimes.com