U.S. flag

An official website of the United States government, Department of Justice.

DNA - A Prosecutor’s Practice Notebook Inventory

Database Challenges

Home  |  Glossary  |  Resources  |  Help  |  Contact Us  |  Course Map

The defense may argue that the way a database was created is flawed.

The NRC II states there is no reason to suspect that persons who contribute to blood banks differ from a random sample of the population. It found that "there is empirical evidence to the contrary: If a comparison is made of samples chosen in different ways, the results from calculations made from the different databases are quite similar" [NRC II, p. 127.].

The NRC 1 suggested that 100 randomly chosen people provide 200 alleles, which are adequate for estimating allele frequencies (page 94).

The defense may argue that the database is too small to be meaningful. 

A statistically sufficient database requires a minimum of only 100 samples. These calculations are based on the number of all possible DNA profiles for the loci used.

The defense may assert the database does not reflect all of the subpopulations represented in the jurisdiction.

First, the database includes the largest subpopulations in a jurisdiction merely to assist in interpreting the DNA results. Humans share 99.9% of their DNA. People differ in only 0.1% of their DNA.

Second, the frequency differences between subpopulations are considered to be statistically insignificant. The NRC II recommends the use of a statistical compensation called the theta correction (NRC II at p. 29-30, 102-106, and 114-116). The theta correction compensates for any of the minor differences in subpopulations.

Frequency Differences

The analyst typically does not know the race of the defendant. If a defendant is from an unusually isolated subgroup and it would be expected that the person committing the crime would be a member of this isolated subgroup, notify the lab and ask that a more conservative correction be used for the statistical calculation in accordance with their lab protocol and NRC II recommendations. (see National Research Council, The Evaluation of Forensic DNA Evidence (1996), p. 116)


Back Forward