Fingerprints and Impressions Questions and Answers

Answer: B

The dynamic process produces striation marks.

Tool marks are of two kinds. When a tool slides or scrapes along a surface, as when a screwdriver is levered or a knife cuts, the fine irregularities of its edge cut a pattern of parallel lines, or striations, which can be matched to the tool.

Compression marks, indentation marks and imprints are static marks, made when a tool is pressed into a softer surface without sliding, as in a hammer blow or a jemmy pressed into a door frame; they record the shape of the tool rather than a pattern of lines.

Answer: C

Tool marks do not consist of elemental composition.

A tool mark is the impression a tool leaves on a surface, and its forms are indentations (impressed or compression marks), striations (sliding marks), and combinations of these such as cutting and drill marks.

Elemental composition is a property of materials: traces of paint or metal transferred between the tool and the surface can be analysed for their elements, but that is trace evidence associated with a tool mark, not a part of the mark itself.

Answer: C

The key accepts statements I, II and III, so the answer is I, II and III are correct.

The pattern a tool leaves depends on how it was used. The horizontal angle and the vertical angle at which the tool meets the surface change which part of the edge makes contact and so the spacing and width of the striations, and the pressure applied changes the depth and clarity of the mark. That is why examiners make test marks at several angles and pressures before comparing them with the questioned mark.

Which side of the blade was used is a feature of the tool's identity rather than one of these working parameters, which is why the key leaves out IV.

Answer: C

The minimum meaningful area under the ROC curve is 0.5.

The receiver operating characteristic curve of a biometric system plots its genuine acceptance rate against its false acceptance rate as the decision threshold changes. The area under the curve measures how well the system separates genuine users from impostors: 1.0 is perfect separation.

An area of 0.5 corresponds to the diagonal line, the performance of random guessing, so it is the minimum a working system can have; a value below 0.5 would mean the decisions were systematically reversed and could be improved simply by inverting them. Values of 0.2, 0.3 and 0.4 are therefore not the minimum.

Answer: D

The Battley lens has seven concentric circles.

In Harry Battley's single-digit classification, devised at Scotland Yard in the 1920s and 1930s to file individual scene marks, a reticle engraved with seven concentric circles, lettered A to G outwards, is placed over the core of a whorl. The circle in which a particular ridge feature, such as the delta or the end of a ridge, falls gives a letter that becomes part of the classification.

Three, five or six circles do not match the lens, whose seven zones give the seven letters A to G.

Answer: D

The final match rests on the comparison of minutiae or ridge characteristics.

Pattern type (loop, whorl, arch), ridge counts and triradii are class characteristics: they let examiners classify and file prints and exclude non-matching ones, but many people share them.

Identification of an individual depends on the minutiae (Galton details), such as ridge endings, bifurcations, dots, islands and enclosures, and on their type, position and relationship to one another. Because these details are unique and permanent, agreement in minutiae with no unexplained difference is what establishes that two prints come from the same finger.

Answer: D

Ridged skin is found in humans, apes as well as monkeys.

Friction ridge skin on the palms and soles is a feature of primates. Apes and monkeys have ridged skin on their hands and feet, and some New World monkeys have it on the underside of their prehensile tails as well. Koalas also have fingerprints remarkably like human ones.

The ridges improve grip and touch sensitivity, which is why they appear in animals that grasp. So the options saying ridged skin is found only in humans, or is absent in apes or monkeys, are wrong.

Answer: A

Such triradii are called extralimital triradii.

A triradius is the point where three ridge systems meet. Sometimes the pattern of the ridges shows that a triradius would form, but it lies beyond the edge of the ridged skin and so does not actually appear on the print. Dermatoglyphics calls such a predicted but absent triradius extralimital, literally 'outside the limit'.

The other terms do not describe this. A triradius that is present is simply a triradius, and 'conjoint', 'super' and 'phantom' triradii are not the standard name for one lying outside the ridged area.

Answer: A

The true statement is that height can be roughly estimated from footprints.

Foot length bears a fairly consistent relationship to stature, at roughly 15 per cent of height, and regression formulae derived for particular populations allow an approximate estimate of height from the length of a footprint or shoe print. Stride length in a trail of prints gives further clues.

Footprints cannot reliably establish race, caste or ethnicity. Average foot shapes vary somewhat between populations, but the overlap is so great that no individual print can be assigned to such a group, and caste is a social category with no anatomical marker at all.

Answer: B

A footwear print on clothing is best lifted with an electrostatic dust-print lifting device.

A shoe print on fabric is usually a dust impression, a two-dimensional deposit of dry dust or soil. An electrostatic lifter lays a thin metallised film over the print and charges it with high voltage; the dust particles are attracted to the film, which can then be photographed under oblique light. It lifts faint dust prints from porous and textured surfaces like cloth that tape cannot handle well.

Plaster of Paris and liquid casting materials are for three-dimensional impressions in soil or snow, not for prints on fabric. Adhesive tape can lift prints from smooth surfaces but tends to pull fibres and lift poorly from cloth.