Forensic Science Research Glossary

CSAFE research and training involves many statistical, forensic science and legal terms that may be unfamiliar to the general public.  We invite you to use the material below as a resource for learning more about the type of work we do.

3D imaging

Recent approaches in 3D imaging take advantage of digital photography. A set of digital images of a footmark or target area of interest taken in different viewpoints is required to create a detailed and accurate 3D model. Appropriate software allows visualization of a 3D model in different ways, allowing accurate measurements from a 3D trace or performing a comparison of multiple traces.

3D Scanning

Footwear evidence recovered with use of 3D scanners.

There are two types of 3D scanners used in forensics:

  • Crime scene scanners able to capture a large overview map of the scene;
  • ‘Close-up’ 3D scanners able to capture individual objects in high resolution and full color.

Advantage of 3D scanning over casting is that 3D scanners can scan the object without touching or affecting it – most 3D scanners use lasers to capture 3D information. Some scanners can also capture the color surface, producing a visually accurate replica of the object. To create a complete and highly detailed 3D model of a footprint less than 15 minutes is required.

Algorithm

Self-contained step-by-step set of operations performed to solve a specific set of problems. Algorithms exist to perform calculations, data processing and automated reasoning

Bayes Factor

Ratio of the likelihood of data under the null hypothesis and the likelihood of the data under the alternative hypothesis. For certain scenarios, the Bayes factor is equal to the likelihood ratio. This, however, is not always the case due to the different prior beliefs one may have under the two hypotheses.

Blind Proficiency Testing

Determining the performance of an individual and/or laboratories during which the participants under evaluation are not aware they are being reviewed.

Breechface

In a firearm this is the front part of the breechblock that makes contact with the cartridge. The breechblock is what holds a round in the chamber and absorbs the recoil of the cartridge when the round is fired, preventing the cartridge case from moving

Bullet Shell Casing

When a gun fires, it ejects the bullet casing, which contains the bullet and the requisite gunpowder. At the very end of the shell casing, marks left from the firing pin of the gun and ejector help determine which gun or type of gun fired the bullet and its casing.

Bullet Striations

Where a bullet is pushed through the barrel of a rifled firearm, spiral grooves cut into the inside of the barrel (rifling) cause the bullet to spin. This allows increased accuracy of the shot. On the way out of the barrel, the bullet also acquires incidental markings, or striations, which may help in identifying the gun which fired a bullet.

Casting

Recovering evidence such as footwear left by 3D impression often uses plaster casting. Isolating the impression begins by framing the area with a solid boundary. Examiners then gently pour a plaster mix inside the frame. In cases where the surface is not ideal for casting, examiners utilize prior techniques to gain a better cast of the impression. Applying an aerosol resin, glue or hair spray fixes sand in place. Pipetting water from the surface and applying hot air in the form of hair dryer dry wet mud impressions.

Class characteristics

Distinctive features shared by many items or people of the same type. This could be a result from a manufacturing process, such as physical size, design and mold characteristics, or for example hair color or blood type. Class characteristics can determine to which group an item belongs but does not discriminate one item in a group from another item in the same group.

Cognitive Bias

Mistake in reasoning, evaluating, remembering or other cognitive process, often occurring because of holding onto one’s preferences and beliefs regardless of contrary information that may affect decision-making.

Fingerprint Analysis

Comparing sets of fingerprints found in the field, known as “latent prints” with those in a database of recorded prints with a known source. Most analysis focuses on the patterns of friction ridges on each finger, and a set of presumably distinctive features known as minutiae.

Fingerprint analysts visually compare the latent print from the crime scene to the fingerprint of a suspect, examining qualities in the two prints. The analyst then relies on his or her experience to make an evaluation of the prints, either to identify a match, exclusion or a lack of sufficient information.

Forensic Inference

Quantifying the weight of evidence in forensic identification

Foundational Validity

Established for a forensic science method shown, based on empirical studies, to be repeatable, reproducible and accurate at levels measured and are appropriate to the intended application.

Inverse Problem

Mathematical framework used to calculate from a set of observations the causal factors that produced them. The solution to this problem is useful because it generally provides information about a physical parameter that not directly observable.

Latent Print

Fingerprint left at a crime scene and remains invisible to the naked eye until chemically treated. Technicians use fingerprint powder, fuming, and other techniques. The importance of latent print evidence is its ability to identify an individual.

Lifting

Footwear impressions can be lifted from surfaces with tools such as adhesive lifters, gelatin lifters or electrostatic lifting devices. Fingerprints can also be lifted using dusting techniques.

Likelihood Ratio

Statistical measurement comparing two hypotheses of interest. Typically, in the application to forensic evidence, the numerator of the ratio expresses the likelihood of finding that evidence under the prosecutor’s hypothesis (that the defendant left the evidence), while the denominator expresses the likelihood of the evidence under the defense’s hypothesis (that the defendant did not leave the evidence). In this construction, smaller values of the likelihood ratio tend to favor the defense, while larger values tend to favor the prosecution.

Quality Metrics

Used when evaluating whether a project is successful by measuring performance against quality standards to determine whether they’re meeting expectations. Quality metrics can reveal weaknesses and enable quick corrections of areas of deficiency.

R

Programming language and software environment for statistical computing and graphics. The R language is widely used among statisticians and data scientists for developing open-source statistical software and data analyses.

R Package

an extension to the R software written by an R user who has created a suite of functions, data, etc. to solve a specific problem, such as fitting generalized mixed effect models, creating advanced graphics or performing steganalysis tasks.

Randomly Acquired Characteristics

Identifying characteristics that do not result from the manufacturing process, but are accidental, unpredictable characteristics that result from wear. These could include objects that have become attached to the outsole of a shoe—such as rocks, thumb tacks, or tape—or marks on a bullet or casing caused by lack of cleaning of a gun.

Shoeprint Analysis

Initially investigators will look to identify the make and model of the shoe or trainer that made an impression. Analyzed visually or by comparison with evidence in a database, both methods focus heavily on pattern recognition, size and brand or logo marks. Information about the footwear comes from the analysis of wear patterns that are dependent on angle of footfall and weight distribution. Detailed examination of footwear impressions can help to link a specific piece of footwear to a footwear imprint due to unique characteristics of each shoe arising after a certain amount of wear. Footwear evidence occurs most often as either footwear impressions left in a soft surface, such as mud or as dust deposits, which are difficult for the human eye to detect. Often, examiners dust and lift prints in a similar way to fingerprints. A person standing in blood and subsequently trailing it as they move around the scene may also leave footmarks.

Statistical Model

Represents the hypothesized or assumed data-generating process for a set of observations. The model embodies a set of assumptions concerning the underlying mechanism assumed to have created or led to the observed data. A statistical model generally applies to other observations from the larger population with some degree of uncertainty to account for the fact that it is usually impossible to observe all members of a population.

Statistical Validity

Refers to whether a statistical study is able to draw conclusions that are in agreement with statistical, mathematical and scientific laws. It refers to the accuracy of a conclusion drawn from a given data set after experimentation. The validity of a measurement tool is considered to be the degree to which the tool measures what it claims to measure.

Steganalysis

Analysis of cover material to identify presence of hidden information in the field of steganography.

Steganography

Practice of concealing a file, message, image, or video within another file, message, image, or video. The former is called the “hidden information” and the latter is referred to as the “cover” or “cover material”.

Subclass Characteristics

Features common to a subset of class members that occur due to the method of manufacturing. For example, all guns manufactured right before sharpening or cleaning the machinery.

Validity as Applied

The method in question reliably applied in practice.

Legal terms

Admissible

Pertinent and proper information considered in reaching a decision. Refers to the evidence considered in determining the issues to be decided in any judicial proceeding.

Convict

To find a person guilty of a criminal charge.

Cross-examination

The examination of a witness upon a trial or hearing, or upon taking a deposition, by the party opposed to the one who put him on the witness stand to testify.

Defendant

The party against whom relief or recovery is sought in a court action or suit. Sometimes used to designate the accused in criminal or traffic cases

Deposition

The testimony of a witness taken upon oral examination, after notice to the adverse party, not in open court, but in pursuance of a notice to take testimony issued by the party wanting the deposition. The adverse party has the right to attend and cross-examine. Testimony reduces to writing and duel authentication and is often used in connection with the trial of an action in court.

Direct Examination

Initial questioning of a witness by the party who called the witness.

Elements

Facts or circumstances that define a crime requiring proof beyond a reasonable doubt in order to result in a conviction.

Evidence

All the means by which a matter of fact, the truth of which is submitted for investigation, is established or disproved.

Jury

A body of persons selected from the community to hear evidence and decide a criminal or civil case.

Perjury

A criminal offense committed by giving a false statement given under oath.

Probative Value

Ability of a piece of evidence to make a relevant disputed point more or less true in a trial.

Testimony

Oral evidence offered by a competent witness under oath, used to establish some fact or set of facts.

Verdict

The formal decision or finding of guilt or innocence in a criminal case.