5 I. Observer Effects and Examiner Bias Chisum and Turvey quote Paul L. Kirk, who was a pioneering criminalist, about the interpretation of evidence, “Physical evidence cannot be wrong, it cannot be perjured, it cannot be wholly absent. Only in its interpretation can there be error.” (Chisum, p. 51). This becomes a real issue because a great deal of the evidence we deal with can be interpreted in various ways depending on a number of subjective influences. A. Observer Effects – Observer effects can be both conscious and subconscious. Both conscious and subconscious needs and expectations shape both our perception of facts and their interpretation. It can affect what is recognized as evidence, what is collected, what is examined, and how it is interpreted. At its most basic, an observer effect is a psychological bias or effect on the observer’s part that distorts how the evidence is recognized, collect, examined, or interpreted. It is often subconscious (below the level of awareness) on the part of the observer and may significantly affect the reconstruction of the crime. We all have them and the question thus becomes, not whether I have them, but how do I guard against them and eliminate their influence on my reconstruction. B. Potential Observer Effects 1. Ambiguity and Subjectivity – Ambiguity is a factor when evidence or circumstances are incomplete, murky, or equivocal. Subjectivity is a factor when identifications and interpretations rest on the examiner’s experiences or beliefs. They become problematic when the examiner or investigator believes that his experience is all that is required to render an identification. There are at least three areas in reconstruction where subjectivity can show up: 1) evidence collection; 2) evidence quantity and quality; 3) lack of standards for qualifying the results of comparative analysis and identification (Chisum, p. 59). The occurrence of ambiguous physical evidence as well as evidence that is susceptible to subjective interpretation opens the way for subconscious observer effects to affect the results you obtain. 2. Lure of Expectation – We as investigators are often put in situations where we have access to information that can give rise to conscious or unconscious expectations. One of the most common expectations of this type is that the subject must be guilty of something even if they are not guilty of the crime of which they are accused. I once had another officer tell me (in reference to a real thug/scumbag that was a suspect in a homicide) that even if he had not done the crime, to charge him would not be a great miscarriage of justice. We work in a pro-prosecution environment where the suspect’s guilt is suspected and anticipated and this may lead to subconsciously developing pre-examination expectations that may influence the results (Chisum, p. 60). 3. Single Sample Testing – Evidence that is turned over to forensic examiners tends to fall into one of three catetor.

1. 5
I. Observer Effects and Examiner Bias
Chisum and Turvey quote Paul L. Kirk, who was a
pioneering criminalist, about the interpretation of evidence,
“Physical evidence cannot be wrong, it cannot be perjured, it
cannot be wholly absent. Only in its interpretation can there be
error.” (Chisum, p. 51). This becomes a real issue because a
great deal of the evidence we deal with can be interpreted in
various ways depending on a number of subjective influences.
A. Observer Effects – Observer effects can be both conscious
and subconscious. Both conscious and subconscious needs and
expectations shape both our perception of facts and their
interpretation. It can affect what is recognized as evidence,
what is collected, what is examined, and how it is interpreted.
At its most basic, an observer effect is a psychological bias or
effect on the observer’s part that distorts how the evidence is
recognized, collect, examined, or interpreted. It is often
subconscious (below the level of awareness) on the part of the
observer and may significantly affect the reconstruction of the
crime. We all have them and the question thus becomes, not
whether I have them, but how do I guard against them and
eliminate their influence on my reconstruction.
B. Potential Observer Effects
1. Ambiguity and Subjectivity – Ambiguity is a factor when
evidence or circumstances are incomplete, murky, or equivocal.
Subjectivity is a factor when identifications and interpretations
rest on the examiner’s experiences or beliefs. They become
problematic when the examiner or investigator believes that his

2. experience is all that is required to render an identification.
There are at least three areas in reconstruction where
subjectivity can show up: 1) evidence collection; 2) evidence
quantity and quality; 3) lack of standards for qualifying the
results of comparative analysis and identification (Chisum, p.
59). The occurrence of ambiguous physical evidence as well as
evidence that is susceptible to subjective interpretation opens
the way for subconscious observer effects to affect the results
you obtain.
2. Lure of Expectation – We as investigators are often put in
situations where we have access to information that can give
rise to conscious or unconscious expectations. One of the most
common expectations of this type is that the subject must be
guilty of something even if they are not guilty of the crime of
which they are accused. I once had another officer tell me (in
reference to a real thug/scumbag that was a suspect in a
homicide) that even if he had not done the crime, to charge him
would not be a great miscarriage of justice. We work in a pro-
prosecution environment where the suspect’s guilt is suspected
and anticipated and this may lead to subconsciously developing
pre-examination expectations that may influence the results
(Chisum, p. 60).
3. Single Sample Testing – Evidence that is turned over to
forensic examiners tends to fall into one of three catetories: 1)
samples taken from the crime scene; 2) samples taken from the
victim; 3) samples provided by the suspect (Chisum, p. 60).
Single sample testing has been shown to directly affect whether
an examiner’s report will associate the suspect’s with the crime
scene or victim.
4. Prescreened Evidence – Even the best and most committed
reconstructionist can be tripped up by having insufficient
evidence. The problem with prescreened evidence occurs when
the reconstructionist is given only a narrow amount of the

3. physical evidence necessary to reach an informed conclusion.
This prescreening of the evidence can come from anywhere –
prosecuting attorneys, defense attorneys, superior officers, other
investigators, etc. In fact, it can come from anyone who wants
the reconstructionist to reach a particular outcome that the
person providing the evidence favors. It is absolutely critical
that we have access to all the evidence and information in a
case, whether it appears relevant or not. That is the only way
we can make an informed and unbiased interpretation.
5. Contradictory Findings – This can occur when you have
more than one forensic expert involved in examining the same
evidence. The problem arises when the evidence and
interpretation are re-examined and one or more of the experts
tailors his conclusions and interpretations so that they match the
others.
6. Selective Re-examination – This can occur when there are
contradictory findings and the prosecutor, defense attorney, or
investigator pushes for a re-examination of the evidence by the
expert whose conclusion is inconsistent with their preferred
theory. The push is for them to re-examine the evidence and
bring their findings into line with the findings of the other
examiners.
C. Neutralizing Observer Effects and Examiner Bias – There
are a number of reforms that can be put in place in forensic
laboratories to help neutralize bias. These are beyond the scope
of this course and our positions as investigators. The number
one thing we can do as investigators is to be aware of the
existence of observer effects, their subconscious nature, and
their effect on how we perceive and interpret the evidence. I
cannot emphasize enough that this requires an absolute
commitment to truth and honesty, no matter the cost or where
they lead. Be aware of your own biases (we all have them) and
guard against them. Never fall in love with a theory and always

4. be ready to modify or discard your theory when evidence
contradicts it. You must have an absolute commitment to
examining and understanding all of the information and
evidence in the case and not just part of it.
II. Practice Standards Related to Crime Reconstruction –
There are certain ethical standards that must be maintained in
relation to crime reconstruction: (Chisum, pp. 116-123)
A. The reconstruction must diligently try to avoid bias;
B. All relevant evidence and information must be considered
in order to perform and adequate reconstruction;
C. The reconstructionist must determine if the evidence is of
sufficient quality to provide the basis for a reconstruction.
D. The reconstructionist must visit the crime scene in person;
E. The reconstructionist must make his conclusions and the
basis for them in writing;
F. The reconstructionist must have an understanding of
science, forensic science, and the scientific method;
G. The conclusions MUST be based on established facts – no
fact may be assumed for the purpose of the analysis;
H. Any conclusions must be valid inferences based on logical
arguments and analytical reasoning;
I. The conclusions must be made through the use of the
scientific method – that is put the conclusions to the test; and
J. Any evidence, data, or findings on which the conclusions
are based must be made available through presentation of the

5. evidence itself or through citation of a source that can be
examined by others.
Even though these standards technically apply to someone
who is declared an expert in crime reconstruction, they also
should apply to us as investigators as we try to reconstruct the
crime through our investigations. The paramount idea is one
that I have tried to practice in every investigation I have ever
done – Seek the Total Truth and Follow It Wherever It May
Lead!! As trite as it may sound, we are the finders of fact and it
is up to others to determine guilt or innocence based on the
facts we find. Therefore we can never afford to manipulate or
shade the facts in any manner. We have to present the total
facts – everything we know about the case to the triers of fact.
I truly believe that homicide investigation is sacred trust that I
can never violate except at great peril to everything I swore to
protect. Enough said!
END OF READING
Chisum, W. Jerry and Brent E. Turvey. Crime Reconstruction.
Amsterdam: Academic Press-Elsevier, 2007.
Assignment One Reading – Introduction to Basic Crime
Reconstruction
Welcome to Basic Crime Reconstruction. Reconstructing a
crime is not a magical process or the result of some supernatural
insight that is available to only a select few. Rather, it is the
result of pain-staking forensic investigation and logical
deduction. This course will not make you a crime scene re-
constructionist. What it will do is to help you take all of the
factors including forensic evidence, witness statements, suspect
statements, and other evidence; and enable you through the

6. process of logical deduction to sequence the events involved in
the crime.
I. What is Crime Scene Reconstruction?
We are going to focus on something we call holistic crime
reconstruction. As defined by W. Jerry Chisum and Brent
Turvey in their book “Crime Reconstruction”, holistic crime
reconstruction “is the development of actions and circumstances
based on the system of evidence discovered and examined in
relation to a particular crime.”
As part of this type of reconstruction we're going to focus
on all aspects of the crime. The shortcoming of many crime
reconstruction methodologies is that they focus on only one
aspect of the evidence such as forensics or witness statements
or the suspect statements, etc.. The idea behind holistic crime
reconstruction is to bring together all of the information about a
particular crime and to use all of that information in the
reconstruction. You must treat all items of information
involved in the crime as interrelated.
II. Forensic Technicians, Forensic Specialists, and Forensic
Generalists
A. Forensic Technician – A forensic technician is trained in
specific procedures related to collecting, packaging, labeling,
and even performing some tests at the crime scene. As long as
the technician knows how the task should be performed, there is
no real need for them to understand the scientific method or the
greater principles of forensic science.
B. Forensic Specialist – A forensic specialist is someone who
has extensive expertise and training in the analysis of physical
evidence in a particular specialty area. An example of this
would a fingerprint examiner in a laboratory or a ballistics

7. expert. They normally are employed in a crime laboratory and
not out on the street like a technician.
C. Forensic Generalist – A forensic generalist is broadly
educated and is trained in a variety of forensic specialties. He
is a broad picture guy – not an absolute expert in any one area,
but has considerable expertise in many different areas. In fact,
you could say that his major area of expertise is in the
interpretation of evidence and how it fits into the crime puzzle.
III. Crime Reconstruction as the Culmination of a Process –
The reconstruction of a crime is the last step in a long and
methodical process that each piece of evidence/information goes
through. The steps in this process and who is normally
responsible for the evidence at each step are:
A. Recognition (detective/investigator/forensic technician) –
This is the recognition that a piece of material or information is
evidence related to the case.
B. Preservation (detective/investigator/forensic technician) –
This is where the necessary steps are taken to preserve the
evidence or information.
C. Documentation (detective/investigator/forensic technician)
– The necessary photos are taken of the evidence and the
necessary notes made.
D. Collection (detective/investigator/forensic technician) –
The evidence is collected and appropriately packaged and
labeled.
E. Transportation (detective/investigator/forensic technician)
– The evidence is transported to the evidence locker/property
room or to the crime laboratory ensuring that chain of custody
is preserved.

8. F. Identification/Classification (forensic specialist) – This is
the practice of categorizing the evidence according to its
characteristics such as weight, color, chemical composition, etc.
An example of this is when the lab identifies the white powder
as cocaine.
G. Comparison (forensic specialist) – This is the examination
of two or more items to establish similarities and dissimilarities
between them. An example is when you ask the lab to compare
a latent print to known fingerprint.
H. Individuation (forensic specialist) – This is the
establishment of the uniqueness of an item through examination
and experimentation to show that no other item is exactly like
the item in question. An example of this would be the DNA
analysis of a blood drop showing that it could not have come
from any one other than the person it matches.
I. Interpretation/Reconstruction (forensic generalist) – This
is where the significance of the evidence or information is
determined and how it fits into the puzzle that is your crime.
Even though the above listed process is primarily slanted
towards physical forensic evidence, many of the steps also
apply to information such as witness testimony that also helps
you put together the puzzle.
END OF READING