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Forensic psychology
Forensic psychology

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1.3 Impact of estimator variables

Figure 4

There are many variables that will influence the accuracy or completeness of the testimony of a witness. These variables are estimator variables and therefore not under the control of legal professionals. Knowing their potential influence does help us judge how much weight to place on the information provided by a witness, and also helps us to evaluate whether one witness might be more accurate than another.

Later, you will hear the initial statements provided by our two witnesses to both DI Bullet and DS Sund. When you listen to the evidence, see if you can apply the knowledge you have gained here in order to evaluate it.

Below is a list of some of the key factors that you might want to keep in mind:

  • Confidence does not necessarily mean accuracy. Despite her best efforts and being sure she was correct, Jennifer Thompson identified the wrong person.
  • Actions may be remembered better than details.
  • Variables such as how far the witness was from the crime, what the illumination was and how long the witness was exposed to the event and the perpetrators can all have an effect on the accuracy of witness testimony.
  • People are generally not good at judging either time or distance – we often overestimate the duration of short events, particularly if the event is complex.
  • We tend to be poor at dating events – forward telescoping means we often assign a date that is too recent.
  • Estimates of height and weight are not accurate – we have a tendency to underestimate above-average characteristics and overestimate below-average characteristics. The witness’ own height and weight might be used as an anchor against which the height or weight of others are estimated. This means it is a good idea to ask for relative judgements, e.g. how tall was the perpetrator in relation to the doorway?
  • The gender of the witness may impact upon the evidence provided, but its influence is by no means clear-cut.
  • Age can have an effect. Over the age of 70, hearing, vision and attention decline, and children will generally provide less information and be less accurate than adults.

You also heard about the potential problems of co-witnessing. This does not fit the estimator/system variable distinction very well, as it is partly, but not entirely, under the control of the police:

  • Co-witnessing – allowing witnesses to talk to one another, including interviewing them together, will lead to considerable convergence in their testimonies. Witnesses can form a memory of something happening from listening and talking to another witness. So, consistency in testimony is to be expected if the witnesses have had a chance to share memories. Such consistency should not be seen as confirming the facts reported.

In addition, remember that the way in which a question is asked can have a dramatic effect on the response of a witness. We’ll look at questioning later on, but for now remember that the question ‘Who is it?’ tends to make a witness select someone from an identity parade because the question implies the perpetrator is present, and therefore, that the witness should choose someone. When evaluating the witness statements, consider how the detectives ask their questions. Are they giving the witness the opportunity to respond based on their own memories, or are they suggesting the answer to the witness?

In the next sections, you’ll find out about the crime committed and hear the witness statements taken by the two detectives, DI Bullet and DS Sund. First take a look at the timeline of the crime and police response:

Timeline of the crime and police response

16:24, Wednesday afternoon – the police receive a number of emergency calls reporting a possible armed robbery in progress.

Three are from people working in office buildings adjacent to where the crime is taking place who report hearing gun fire; one is from a pedestrian passing by the end of the road who sees an armed, masked man run past; and one is from a car driver who narrowly avoids a collision with a speeding car.

16:26 – An initial response unit is dispatched to the location reported in the emergency calls.

16:32 – The police receive a call from a mobile phone from two eyewitnesses to the crime, who confirm an armed robbery has taken place and also that the robbers kidnapped a young woman, called Liz, a friend of the witnesses who was with them when the crime started.

16:36 – The initial response unit arrives at the crime scene. The two eyewitnesses who made the call are still there, but the robbers have clearly left. The street in which the crime took place is cordoned off.

16:48 – A detective from the Criminal Investigation Department (CID), Detective Inspector Jake Bullet, arrives at the crime scene and, after a brief conversation with the officers already there, talks to and obtains an initial statement from the two eyewitnesses.