The Scientific Working Group on Friction Ridge Analysis, Study and Technology (SWGFAST) has a sufficiency graph for fingerprint comparison that may be useful for attorneys to use when discussing fingerprint comparisons with experts.
The sufficiency graph reflects the interplay between quality and quantity of minutiae. Minutiae are small details that are used in comparing one print to another, such as ridge endings, bifurcations, and dots. These may also be called points of comparison or features of a print. This interplay between quality and quantity determines the level of complexity of a latent fingerprint. By using this graph, a fingerprint examiner should be able to explain her determination of whether a latent print could be individualized to a source.
In the graph above, the solid curve in the graph defines the lower limit of the sufficiency of friction ridge details below which, in area marked A, an individualization decision is not warranted. The dotted curve indicates the boundary between complex and non-complex prints. In area marked B in the graph, the examination is considered as complex and an individualization may be warranted. In area marked C, the examination is considered as non-complex and an individualization is warranted. See SWGFAST, Document #10, Standards for Examining Friction Ridge Impressions and Resulting Conclusions § 22.214.171.124 and § 126.96.36.199 (quotations omitted). Available here.
Additionally, the rarity of the features is factored into the examiner’s analysis and may adjust the position on the graph upward or downward on the graph between the zones.
An attorney could use this graph to discuss a comparison with a fingerprint examiner and arrive at a more concrete understanding of the level of complexity of the comparison. This graph was generated based on the collective experience of members of SWGFAST, not based on any scientific study. So, the curve is not intended to set a fixed requirement about number of points/minutiae, but it might be a starting point for a conversation. For example, if the print has 2 minutiae and low quality, the examiner would have some explaining to do about any individualization made to the source of the print. But, if the print is in a border area, then the examiner and the attorney could discuss why the examiner reached a certain conclusion or not.
Attorneys should read the entire SWGFAST Standards document for a better understanding of how examiners compare latent prints and make decisions about individualization. An understanding of the SWGFAST procedures and the procedures of the examiner’s own lab will improve communication between an attorney and an examiner and will help an attorney evaluate the forensic evidence in the case.