Monday, April 7, 2008

The COmbined DNA Index System CODIS CONCEPT

The COmbined DNA Index System, CODIS, blends computer and DNA technologies into an effective tool for fighting violent crime. The current version of CODIS uses two indexes to generate investigative leads in crimes where biological evidence is recovered from the crime scene. The Convicted Offender index contains DNA profiles of individuals convicted of felony sex offenses (and other violent crimes). The Forensic index contains DNA profiles developed from crime scene evidence. CODIS utilizes computer software to automatically search these indexes for matching DNA profiles.

The word "index" in COmbined DNA Index Systems is not arbitrary. CODIS is a system of pointers; the database only contains information necessary for making matches. Profiles stored in CODIS contain a specimen identifier, the sponsoring laboratory's identifier, the initials (or name) of DNA personnel associated with the analysis, and the actual DNA characteristics. CODIS does not store criminal history information, case-related information, social security numbers or dates-of-birth. Matches made among profiles in the Forensic Index can link crime scenes together; possibly identifying serial offenders. Based on a match, police can coordinate separate investigations, and share leads developed independently. Matches made between the Forensic and Convicted Offender indexes ultimately provide investigators with the identity of the suspect(s).

CODIS also supports a Population file. The Population file is a database of anonymous DNA profiles used to determine the statistical significance of a match.

CODIS is designed so that forensic laboratories have control over their own data. The system has three tiers (or levels): local, state, and national (fig. 1). The forensic and convicted offender indexes, and the population file may exist at each tier.

Typically, the Local DNA Index System, or LDIS, is installed at crime laboratories operated by police departments, sheriff's offices, or state police agencies. At the local level, DNA examiners use CODIS software on the bench when sizing autoradiograms. After sizing, examiners transfer unknown subject profiles into the local forensic index, where they are searched against other unkown subject profiles. The custodian of the local database can share this data with other CODIS labs within the state by forwarding it to the state level.

Each state participating in the CODIS program has a single State DNA Index System (SDIS). The SDIS is typically operated by the agency responsible for implementing the state's convicted offender statute. At the state level, inter-laboratory searching occurs. That is, the DNA profiles submitted by different laboratories within the state are compared against each other. Forensic profiles developed at local laboratories are also searched against the Convicted Offender index. The state custodian can share this data with the rest of the CODIS community by forwarding it to the national level.

The National DNA Index System, or NDIS, is operated by the FBI. NDIS provides a mechanism for forensic crime laboratories located throughout the United States to share and exchange DNA profiles. The DNA Identification Act of 1994 formalized the FBI's authority to establish a national DNA index for law enforcement purposes.

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