Measuring rarity of combined features in palm versus finger impressions and evaluating examiner perceptions

Friction ridge examiners (FREs) consider the rarity of specific minutia types when making decisions about the suitability of an impression (whether there is enough reliable information visible to proceed to a comparison) or the sufficiency of a proposed association (whether there is enough reliable information between two impressions to support a source conclusion). It is generally accepted in the friction ridge community that when features of higher rarity are observed, fewer are needed to support suitability and sufficiency decisions. But how does an FRE know which features are rarer than others or how much rarer they are? Typically, FREs refer to a “mental database” made up of the features they have observed during their career to guide them. However, because fingers are encountered with greater prevalence than palms in casework, this mental database is skewed toward fingerprint data. No published studies have measured the relative frequencies of minutiae (bifurcations and ridge endings) or combined minutia types (e.g., spurs, enclosures, ridge breaks) in palms. Because FREs receive less training and perform fewer comparisons on palms, and because there is less research on palms than fingers, it is likely that FREs use the heuristic of assuming that anything that is true of fingers must be analogous in palms. This may result in FREs applying the same relative feature frequencies they use for finger impressions to their palm comparisons. The risk of doing this is that features in palms may be over- or underweighted when making suitability and sufficiency decisions if these values actually differ. In their previous palm research, the proposed research team has noted that many of these combined features appear with higher frequency in palms than in fingers. We propose to sample finger and palm impressions to establish evidence-based relative frequencies of ridge endings, bifurcations, and a slate of combined minutiae. We will then survey FREs to evaluate their perceptions of these relative frequencies and assess whether calibration is needed to the subjective weights assigned to these features in fingers and palms. Such a calibration step is critical to ensure a sound basis to the holistic expertise deployed by FREs and to allow expressions of relative rarity that are supported by empirical and defensible data. We will provide data on which FREs can rely in training and in court when invoking the rarity of features, and we will disseminate our findings through publication, presentations, and training materials.

Note: This project contains a research and/or development component, as defined in applicable law," and complies with Part 200 Uniform Requirements - 2 CFR  200.210(a)(14). CA/NCF