Solving the spatter is no small feat...

GRAHAM WILLIAMS, lecturer in forensic science and consultant forensic biologist looks at how blood stains can help solve crimes

Recently I met an old friend; upon telling him that I was a forensic biologist, he immediately asked ‘Is that where you do that thing with blood and string or lasers?’

My response to this question frequently changes, usually depending on what mood I am in! While it is correct that Bloodstain Pattern Analysis (BPA) is an area in which I am an expert witness, I do not play around with string or lasers. It is easy to see why such a question arises given the prevalence of forensic themed TV shows such as “CSI” and “Dexter”.

In such programmes, if a bloody, gruesome crime has been committed, then usually the red string or red lasers are deployed and a Sherlockian interpretation is carried out.

There is a sound principle behind the use of “stringing”. When blood becomes airborne, it quickly forms a spherical shape.

If that blood drop then lands on a surface at an angle that is perpendicular to the surface, (in other words, that the angle between the flight path of the blood and the surface is 90 degrees) then the resulting blood stain is perfectly circular.

If the angle between the flight path and surface (angle of impact) is less than 90 degrees, then the shape becomes elliptical. As the angle of impact decreases, the shape of the blood drop becomes increasingly elliptical.

It is possible to work out the angle of impact from measuring the blood stain.

This is where “stringing” comes in. You get a piece of string and attach one end to the surface on which the blood stain lies.

Using a protractor you can then adjust the string to the calculated angle of impact.

If this is carried out on a large number of stains, then it is possible to identify an area of origin of the blood stains — or at least in theory.

The problem with this technique, however, it only works if the blood drop travels in a straight line.

Unfortunately this is not the case, once the blood drop becomes airborne, gravity acts on it, which means that the blood drop is actually travelling in a curve; in a similar manner as a thrown tennis ball.

This makes the stringing method unreliable.

Actually, in real case work, the stringing method is hardly ever used.

So what is the use of BPA? It is a very useful forensic discipline, as long as it used appropriately. BPA evidence is most powerful when used in what is known as a Bayesian Framework.

Simply put, it is the evaluation of the evidence given two mutually exclusive hypotheses. Such examples of mutually exclusive hypotheses include “guilty” or “not guilty”.

This can be more appropriately framed as being “suspect punched and kicked the victim” and “suspect did not punch and kick the victim”.

If the first hypothesis was true, then we might expect to find the victim’s blood on the suspect’s clothing. If the second hypothesis was true, then we might not expect to find such evidence.

So in this particular case, finding blood would support the first hypothesis.

However, if the alternative hypothesis was “suspect did not punch and kick victim, but was a bystander whilst someone else did it” then we might still expect to find blood on the suspect’s clothing.

This is where BPA is then utilised.

Bloodstains can be broadly divided into airborne stains or “spatter” and contact or “non-spatter” stains.

The latter being caused by the source of the stain coming into direct contact with the surface of interest.

If the first hypothesis was true then we might expect to find both airborne stains and contact stains.

If the suspect was merely a bystander, then we might expect to find airborne stains, but we might not expect to find contact stains.

Therefore, if we examine the suspect’s clothing for blood and we find contact stains, then this would support the view that he punched and kicked the victim.

If we do not find contact stains and we do find airborne stains, then this supports the view that the suspect was actually a bystander and should be released.

This shows that in a lot of the cases, the presence of airborne stains, in which “stringing” could be carried out, does not actually help solve the crime.