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 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.
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)
The National Research Council was convened in 1989. In 1996 the NRC filed the report The Evaluation of Forensic DNA Evidence (referred to as NRC II). This document revised and expanded the initial report written in 1992 (NRC I). These recommendations are recognized by U.S. courts.
Additional Online Courses
- What Every First Responding Officer Should Know About DNA Evidence
- Collecting DNA Evidence at Property Crime Scenes
- DNA – A Prosecutor’s Practice Notebook
- Crime Scene and DNA Basics
- Laboratory Safety Programs
- DNA Amplification
- Population Genetics and Statistics
- Non-STR DNA Markers: SNPs, Y-STRs, LCN and mtDNA
- Firearms Examiner Training
- Forensic DNA Education for Law Enforcement Decisionmakers
- What Every Investigator and Evidence Technician Should Know About DNA Evidence
- Principles of Forensic DNA for Officers of the Court
- Law 101: Legal Guide for the Forensic Expert
- Laboratory Orientation and Testing of Body Fluids and Tissues
- DNA Extraction and Quantitation
- STR Data Analysis and Interpretation
- Communication Skills, Report Writing, and Courtroom Testimony
- Español for Law Enforcement
- Amplified DNA Product Separation for Forensic Analysts