THE COMPARISON OF EVIDENCE

Definition of terms:

Class characteristics

Characteristics that enable an object to be placed into a particular category, for example identifying a trainer as belonging to a certain brand

Test impression

An impression deliberately made using a suspect item in order to compare it with a scene impression.

Scene impression

An impression detected at the scene of a crime, which may be of potential forensic importance.

Individual characteristics

Characteristics that are unique to a particular object (e.g. a tool, tyre or shoe) and, as such, are potentially useful in the identification of scene impressions

COMPARISON BETWEEN AN EVIDENTIAL OBJECT AND A RELEVANT DATABASE

In some instances, the purpose of this type of comparison is to identify a category to which an item of evidence belongs. To achieve this, the class characteristics of the evidential item concerned are established. For example, if footwear impressions or prints are recovered from a crime scene, their sole patterns may be established and then these may be usefully compared with sole patterns held on a footwear database

Through this exercise, it may be possible to identify the manufacturer and, conceivably, the style of the shoe concerned. This type of footwear comparison is particularly relevant to trainers. Similarly, tyre marks left at an incident scene may be compared with an appropriate database of tread pattern designs.

With some specific types of forensic evidence, namely fingerprints and samples of body fluids or tissues used for DNA profiling, the object of comparison with a database is the identification of the individual concerned.

COMPARISON BETWEEN TWO PIECES OF EVIDENCE OBTAINED FROM DIFFERENT PLACES

This type of comparison seeks to determine whether two pieces of apparently similar forensic evidence, for example hairs, textile fibres, paint chips or glass fragments, may share a common origin. Its purpose, therefore, is to determine whether any possible link exists between the two separate locations from which the evidence has been retrieved. This may be between two individuals (as in the case of the victim of an attack and his or her assailant), between an individual and a crime scene, or even between two different crime scenes. This type of comparison may be usefully illustrated by the following hypothetical scenario.

Consider a case in which a car window is broken and the CD player stolen from the vehicle. A suspect is apprehended by the police and, although the CD player is not in the suspect’s possession, there are splinters of glass adhered to the right-hand cuff of his jacket. A comparison is made between shards of glass taken from the car window and those recovered from the suspect. If these samples are found to be indistinguishable, this provides evidence that is consistent with the suspect being at the crime scene.

COMPARISON BETWEEN QUESTIONED SAMPLES, BOTH POSITIVE AND NEGATIVE CONTROLS, AND REFERENCE COLLECTIONS

A crime scene sample that is to be tested to find its evidential value is usually referred to as a questioned sample (or sometimes a disputed sample). Such tests are designed to evaluate a hypothesis. A hypothesis is a supposition that is either true or false and that can be tested by experimentation. For example, if a suspect is detained and found to possess a packet containing a pale brown powder, then the hypothesis may be that the powder is heroin. In order to test this hypothesis, experiments may be carried out that compare the chemical characteristics of this questioned sample with those of a known sample of heroin. Known samples such as this are referred to as positive controls, known or standards. If the questioned sample and the positive control are shown to have characteristics in common, it might be concluded that the questioned sample is indeed heroin. However, this may not be the case. It is possible that the chemicals and/or equipment used in the test were contaminated with heroin. In order to eliminate this possibility, it is necessary to carry out the test in a way that is identical in all respects to the tests to be carried out on the questioned sample and the positive control sample, except that it contains neither of these materials. Such a test is known as a negative control or a blank. In some instances, it is necessary to go to considerable lengths when carrying out the negative control test. For example, when testing for trace levels of explosives, swabs from all surfaces that will come into contact with the sample will be obtained. These will then be tested to show that the equipment was free from explosives. Note that in many applications, the term ‘control’ is used to denote either positive or negative controls; the context makes it clear which type of control is being referred to. There are circumstances in which it is valuable to compare a questioned sample with a number of positive controls. For example, the properties of a liquid retrieved from a scene of suspected arson may be compared with those of a range of flammable liquids, such as different types of petrol, paraffin and diesel fuel. Through comparison, it may be possible to identify the questioned sample via elimination and positive matching. A collection of positive controls used for such a purpose is known as a reference collection.

COMPARISON BETWEEN A SCENE IMPRESSION AND A TEST IMPRESSION

Impressions made by recognizable objects, such as footwear, tyres and tools, are often detected during the examination of crime scenes. If an object suspected of creating the impression(s) in question is subsequently discovered, then that object may be used to create a series of test impressions. A comparison of these test impressions with the scene impression(s) may reveal that both types were created by objects with the same class characteristics. However, in some cases, it may be possible to proceed beyond this stage and identify the suspect item as being the actual one used in the commission of a crime. This can occur when individual characteristics, namely those that are peculiar to a particular individual object, are shown to be visible on the scene impression(s), as well as on the test impressions. Such individual characteristics may be created by some aberration during the manufacturing process but are more likely to be acquired as a result of general wear and tear. Characteristics that are exhibited in evidence and that are capable of identifying a specific item are said to be individualizing.

FORENSIC EVIDENCE COLLECTION

Investigators should perform the evidence collection process in a systematic and careful manner.

The process begins with the preliminary crime scene survey/walk-through, followed by a determination of the evidence collection sequence to be used. 

The evidence collection sequence may be based on the following information:

• The scene location:  interior, exterior, within a vehicle.

• The condition of the evidence: either fragile or stable.

• Weather conditions which might affect the scene or evidence within.

• Scene management considerations which may alter or contaminate the evidence.

• Additional processing techniques that may need to be conducted at the scene with specialized personnel.

Investigators should use the appropriate equipment when collecting evidence.  Collection equipment that may come into contact with evidence should be sterile. 

The following equipment may be used in the evidence collection process:

• Latex gloves/nitrile gloves (N-DEX, non-latex).

• Forceps.

• Tweezers.

• Scalpels.

• Swabs.

• Paper bags.

• Plastic bags.

• Cardboard boxes.

• Wrapping paper.

• Hand tools.

• Thermometer.

• Plastic 5 gallon bucket with lid.

COLLECTION METHODS

The swabbing collection technique should be used for the recovery of biological evidence in a dried or liquid state. Best practice techniques include the following:

DRIED MATERIAL COLLECTION TECHNIQUE

• With gloved hands, slightly moisten the swab with distilled water. (The swab should be damp but not overly wet.)

• Thoroughly rub the stained area using a single moistened swab for a small stain and multiple swabs for a large stain.  When only a small amount of the stain is available, concentrate as much of the stain as possible on the tip of the swab.

• Air-dry the swabs.

• Place each swab into separate package.

• This package may be placed inside a paper envelope.

• Collect a substrate/control sample from an unstained area using the same techniques.

LIQUID MATERIAL COLLECTION TECHNIQUE  

A. When suspected biological evidence is found on clothing or other absorbent surfaces transport it to the laboratory in an appropriate container.  Wet evidence should not be folded over on itself. Use paper wrapping to prevent contamination during the transfer. This will protect bloodstain patterns and prevent cross-contamination between stains on one item. The item should be air-dried thoroughly in a drying locker and packaged in a container suitable for dried evidence. 

B. If the suspected biological evidence is in a liquid form on a fixed surface that cannot be transported (i.e., concrete floor), the substance should be recovered using the following swab technique:

1. With gloved hands, swab the liquid material allowing the swab to absorb as    much of the substance as possible.  Multiple swabs should be obtained when a large quantity is available. 

2. Thoroughly air-dry each swab.

              Package the swab inside an appropriate container.

3. Collect a substrate/control sample from an unstained area using the same       techniques.

Bloodstain Pattern Analysis

LOW, MEDIUM AND HIGH VELOCITY BLOOD SPATTER AND APPLICATION IN FORENSIC SCIENCE

INTRODUCTION

Bloodstain Pattern Analysis is the examination of the shapes, locations, and distribution of patterns of bloodstains, in order to provide an interpretation of the physical events that gave rise to their origin.

Terms used in Blood Stain Pattern Analysis

1.      Angle of Impact: the angle at which a blood droplet strikes a surface

2.      Arterial Gushing: the large pattern of blood that is created when blood escapes an artery under pressure; the increase and decrease in blood pressure is apparent

3.      Arterial Spurts: large patterns created under pressure, but with less volume and usually more distinctive evidence of blood pressure rising and falling

4.      Clot: a mass of blood and other contaminants caused through clotting mechanisms

5.      Cast-Off Stains: blood that has been thrown from a secondary object (weapon or hand) onto a target other than the impact site

6.      Drop Patterns: characteristic patterns present when blood drips into standing, wet blood

7.      Expiratory Blood: blood which is spattered onto a target, as a result of breathing; typically, this occurs when an injury is sustained to the throat, mouth, or airway

8.      Impact Site: usually the point on the body that received the blow or applied force, from which the blood was shed

9.      Origin: the point in space where the blood spatter came from

10. Parent Drop: the droplet from which satellite spatter originated

11. Projected Blood: blood under pressure that strikes a target

12. Satellite Spatters: small drops of blood that break off from the parent spatter when the parent droplet strikes a target surface

13. Shadowing/Ghosting/Void: a pattern that helps to place an object or body in the scene; normally, the area in question lacks blood even though areas surrounding it show blood

14. Skeletonized Stain: the pattern left when an object moves through a partially dried stain, removing part of the blood, but leaving the outline of the stain intact

15. Spatter: bloodstains created from the application of force or energy to the area where the blood is

16. Spines: the pointed edges of a stain that radiate out to form the spatter

17. Splash: pattern created when a volume of blood in excess of 1 mL strikes a surface at a low to medium velocity

18. Swipe: the transfer of blood onto a target surface by a bloody object that is usually moving laterally

19. Transfer Pattern: the pattern created when a wet, bloody object comes in contact with a target surface, leaving a pattern that has the features of the object making it useful for identifying the object

20. Target: the surface where the blood ends up

21. Wipe: pattern created when a secondary target moves through an existing wet blood stain on some other object

Information obtained from a proper Bloodstain Pattern Analysis

1. Distance from the blood source to the target
2. Direction of travel and impact angles
3. Nature of the force used to cause the bloodshed
4. The object used to cause the bloodshed
5. Sequencing of multiple bloodshed events
6.Interpretation of contact or transfer patterns

When properly documented, bloodstain patterns found at the crime scene, or on a particular person's clothing, can be used to:
1. Confirm or refute the position of a victim, witness, suspect, or defendant
2. Determine if there is evidence of a struggle, or if the assault is "one sided"
3. Confirm or refute statements made by principles in the case: i.e. are stain patterns on a particular person's clothing consistent with accounts given by the victim, witness, or defendant?

SPATTER VS TRANSFER

The simplest type of blood spatter analysis is determining spatters from transfers. Spatters are created when blood is acted upon by force, and travels through the air before landing on a target surface. Transfers occur when a blood source comes in direct contact with a target surface area.

LOW FORCE (VELOCITY) IMPACT SPATTER/PASSIVE DROPS:

• Blood that falls at the speed or force of normal gravity

• These spatters usually fall from an open wound, or from a  surface that is saturated with blood
• The majority of the Low Force Impact Spatters are large,  circular, spatters with diameters of 4mm or more

• Low Force Impact Spatters will increase in size as the  distance fallen increases, however, the size of the spatters  will remain constant after approx. 4 feet

MEDIUM FORCE (VELOCITY) IMPACT SPATTER

• Produced with more energy or force than gravity
• The force of the impact causes the blood to break into  smaller size spatters  relative to the amount of  force applied
• This type of spatter is usually seen in blunt force, stabbings,  and secondary spatters
• Produced when the majority of larger drops of blood are  broken into smaller spatters with diameters of 2 – 4 mm
• The force associated with this type of spatter is greater than  25 ft. per second

HIGH FORCE (VELOCITY) IMPACT SPATTER:

• Impact spatter that measures less than 2mm in  diameter
• The force necessary to produce this size spatter  is greater than 100 ft. per second
• This type of spatter is usually associated with  gunshots, explosions, and high speed collisions
• High Force Impact Spatter takes on a "mist like"  appearance

CONCLUSION

It is important to note that the term "Velocity" does not measure the speed at which the blood is traveling, but rather is used to describe or measure amount of force applied to the blood, to cause it to spatter.

Blood will not break up unless it is acted upon by force. The force must be great enough to overcome the surface tension of the blood

Blood forms a spherical shape (perfect circular shape) almost immediately upon separating from the blood source. The spherical shape is caused by the surface tension of the blood.

Surface Tension causes the blood drop to pull itself in; both horizontally and vertically. The blood drop will settle into a spherical shape, as a result of the surface tension.

The surface tension will maintain the sphere shape of the blood drop until it impacts with the surface.