From data to a validated score-based LR system: A practitioner's guide.

Likelihood ratios (LRs) are a useful measure of evidential strength. In forensic casework consisting of a flow of cases with essentially the same question and the same analysis method, it is feasible to construct an 'LR system', that is, an automated procedure that has the observations as input and an LR as output. This paper is aimed at practitioners interested in building their own LR systems.

Automated interpretation of comparison scores for firearm toolmarks on cartridge case primers.

An automated approach for evaluating the strength of the evidence of firearm toolmark comparison results is presented for a common source scenario. First, comparison scores are derived describing the similarity of marks typically encountered on the primer of fired cartridge cases: aperture shear striations as well as breechface and firing pin impressions. Subsequently, these scores are interpreted using reference distributions of comparison scores obtained for representative known matching (KM) and known non-matching (KNM) ballistic samples in a common source, score-based likelihood ratio (LR) system.

Measuring the rarity of core-delta distances in fingerprint patterns in the Dutch population.

In latent print examination, a conclusion on an individualization is primarily based on the presence of corresponding minutiae. Depending on the level of correspondence between the minutiae on the fingermark and on the reference print, a match is concluded. However, a fingermark is analyzed on all three levels: general pattern combined with core-delta distance, minutiae, and third-level detail.

The evidential strength of a combination of corresponding class features in tire examination.

Tire marks are an important type of forensic evidence as they are frequently encountered at crime scenes. When the tires of a suspect's car are compared, the evidence can be very strong if so-called 'acquired features' are observed to correspond. When only 'class characteristics' such as parts of the tire pattern are observed to correspond, it is obvious that many other tires will exist that also correspond, and so this evidence is usually considered very weak or is simply ignored.

Why calibrating LR-systems is best practice. A reaction to "The evaluation of evidence for microspectrophotometry data using functional data analysis", in FSI 305.

In their paper "The evaluation of evidence for microspectrophotometry data using functional data analysis", in FSI 305, Aitken et al. present a likelihood-ratio (LR) system for their data. We show the values generated by this system cannot be interpreted as LRs: they are ill-calibrated and should be interpreted as discriminating scores.

The influence of motion artefacts on magnetic resonance imaging of the clavicles for age estimation.

Purpose: To determine how motion affects stage allocation to the clavicle's sternal end on MRI.

Materials And Methods: Eighteen volunteers (9 females, 9 males) between 14 and 30 years old were prospectively scanned with 3-T MRI. One resting-state scan was followed by five intentional motion scans.

Influence of Muzzle Instability on Bullet Deflection after Perforating Laminated Particleboards.

The influence of muzzle instability on bullet deflection, when perforating laminated particleboards, was studied with three different handgun bullet types. The mean deflection angles of .32 Auto FMJ and .

Measuring the Rarity of Fingerprint Patterns in the Dutch Population Using an Extended Classification Set.

Latent print examiners often use their experience and knowledge to reach a conclusion on the identity of the source. Their conclusion is primarily based on their personal opinion on the rarity of the matching fingerprint features. Fingerprint patterns, if present, can play a significant role in the final assessment of a match.

Probability intervals of speed estimations from video images: The Markov Chain Monte Carlo approach.

Closed Circuit TV (CCTV) systems often record vehicle motion prior to incidents. From the footage an estimate of the average speed of the vehicle between two frames can be calculated. In a forensic investigation not only an estimate of the average speed is needed but also an estimation of the measurement error.

Implementation and assessment of a likelihood ratio approach for the evaluation of LA-ICP-MS evidence in forensic glass analysis.

For the comparative analysis of glass fragments, a method using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) is in use at the NFI, giving measurements of the concentration of 18 elements. An important question is how to evaluate the results as evidence that a glass sample originates from a known glass source or from an arbitrary different glass source. One approach is the use of matching criteria e.

Magazine Influence on Cartridge Case Ejection Patterns with Glock Pistols.

In this study, the cartridge case ejection patterns of six different Glock model pistols (one specimen per model) were compared under three conditions: firing with a loaded magazine, an empty magazine, and without magazine. The distances, covered by the ejected cartridge cases given these three conditions, were compared for each of the six models. A significant difference was found between the groups of data for each of the tested specimens.

Performance Study of a Score-based Likelihood Ratio System for Forensic Fingermark Comparison.

In this article, the performance of a score-based likelihood ratio (LR) system for comparisons of fingerprints with fingermarks is studied. The system is based on an automated fingerprint identification system (AFIS) comparison algorithm and focuses on fingerprint comparisons where the fingermarks contain 6-11 minutiae. The hypotheses under consideration are evaluated at the level of the person, not the finger.

Quantifying Uncertainty in Estimations of the Total Weight of Drugs in Groups of Complex Matrices: Using the Welch-Satterthwaite Equation.

In this paper, a method is described to quantify estimations of the total amount of drugs in groups of seized items, including quantification of the precision. Previous work on this topic was based on the assumptions of normally distributed measurements and grouping of items with a common relative standard deviation. In practice, these assumptions are often violated, for example, for data with point masses at 0, or if certain items in a group have a very high standard deviation.

Numerical likelihood ratios outputted by LR systems are often based on extrapolation: When to stop extrapolating?

A recent trend in forensic science is the development of objective, automated systems for the comparison of trace and reference material that give as output numerical likelihood ratios (LRs). For well discriminating LR systems, often the probability of the evidence given one or the other hypothesis depends on the density from the tail of a probability distribution. The models for probability distributions are trained by data.

Posterior distributions for likelihood ratios in forensic science.

Evaluation of evidence in forensic science is discussed using posterior distributions for likelihood ratios. Instead of eliminating the uncertainty by integrating (Bayes factor) or by conditioning on parameter values, uncertainty in the likelihood ratio is retained by parameter uncertainty derived from posterior distributions. A posterior distribution for a likelihood ratio can be summarised by the median and credible intervals.

The Influence of Wood Grain on the Bullet's Ricochet Behavior.

When a bullet ricochets from wood, various parameters will influence its behavior. In this study, the influence of the wood grain on the ricochet angle (β) and deflection angle (γ) is assessed. Series of five .

An empirical study on the relation between the critical angle for bullet ricochet and the properties of wood.

The properties of a bullet, an object, and the incidence angle determine whether the bullet will penetrate, perforate, or ricochet. In this study, the critical angle for ricochet was established for .32 Auto and 9 mm Luger bullets on Abachi, Southern Yellow Pine, Beech, and Ipe wooden boards.

Quantifying uncertainty in estimations of the total weight of drugs in groups of complex matrices.

In case practice at forensic drug departments, multiple items from one seizure are sometimes sent in with the question: what is the total amount of drugs in the seizure? This may be complicated especially if impregnated material is involved such as clothes or rubber. Measurement uncertainty is typically stable on drug percentages, not weights, and subsampling may take place. It is recognized more and more that determination and reporting of uncertainty on estimators are an essential part of obtaining scientifically sound results in the forensic field.

Fingermark evidence evaluation based on automated fingerprint identification system matching scores: the effect of different types of conditioning on likelihood ratios.

In recent studies, the evidential value of the similarity of minutiae configurations of fingermarks and fingerprints, for example expressed by automated fingerprint identification systems (AFIS), is determined by likelihood ratios (LRs). The paper explores whether there is an effect on LRs if conditioning takes place on specified fingers, fingerprints, or fingermarks under competing hypotheses: In addition, an approach is explored where conditioning is asymmetric. Comparisons between fingerprints and simulated fingermarks with eight minutiae are performed to produce similarity score distributions for each type of conditioning, given a fixed AFIS matching algorithm.

Measurement uncertainty when estimating the velocity of an allegedly speeding vehicle from images.

Sometimes the question arises whether it is possible to estimate the velocity of a speeding car recorded by closed circuit television cameras. By estimating the travelled distance of the car between two images and the time elapsed, estimation of the velocity is rather straightforward. However, to quantify the corresponding measurement uncertainty, the data analysis becomes more involved.

Comparison of the performance of two methods for height estimation.

In the case study, two methods of performing body height measurements in images are compared based on projective geometry and 3D modeling of the crime scene. Accuracy and stability of height estimations are tested using reconstruction images of test persons of known height. Given unchanged camera settings, predictions of both methods are accurate.

Comparison of frequentist methods for estimating the total weight of consignments of drugs.

A topic in forensic statistics is the estimation of the total weight of consignments of drugs based on subsamples of which a certain fraction may not contain drugs at all. The frequentist approach to this concentrates on obtaining confidence intervals for the total weight, based on estimation of the fraction of drugs and the mean and variance of the weights of drug units. The current study shows that the resulting confidence intervals are basically unreliable, since they are based on an underestimation of the variation of the underlying statistical process.

A hierarchical model for body height estimation in images.

In forensic practice, validation experiments performed on known items or persons are used to make predictions on unknown ones. An example of this is body height estimation in digital images. Using a hierarchical statistical model in this case is quite natural as it allows outcomes of the experiment to depend on random effects for test persons and on fixed effects for operators performing the measurements.