Introduction to Forensic Science - UIU

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Laboratory Exercises in Forensic Science

Companion to the text: CRIME LAB: A Guide for Nonscientists by John Houde ISBN 096582862X Calico Press, LLC.

Introduction to Forensic Science

“It is a capital mistake to theo- Part of the Bigger Picture
rize before one has data. Insen-
sibly one begins to twist facts to The term forensic science has a broad meaning,
suit theories instead of theories encompassing many different disciplines as they ap-
to suit facts.” ply to legal issues. Figure 1 shows just a few of the many
subspecialities that might be included under the um-
Sherlock Holmes —Sir Arthur brella of forensic science.
Conan Doyle
One of those subspecialities, criminalistics, re-
quires the practical use of science within the legal sys-
tem to solve problems involving physical evidence.
Physical evidence is that which can be collected and

tested, as opposed to testimonial evidence, for
example. The criminalist, one who
practices criminalistics, may
be involved in a variety of
different areas of the
criminalistics field
or restricted to

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only one area for their entire career. He or she may be Caution
involved in blood alcohol analysis, toolmark compari-
sons, analysis of powders, pills, botanicals, and liquids The criminalist must be careful in their note tak-
for drugs of abuse, or the DNA profiling of a blood spot. ing and report writing not to mix observations with their
Or, he or she might perform a fiber comparison or be interpretations, conclusions or theories. We avoid stat-
asked to assist with crime scene reconstruction or sci- ing an observation as a conclusion. The reader may
entifically answer any question involving physical evi- become confused or misled if the conclusions are not
dence. Depending on the crime and crime scene, each noted separately from the observations. Additionally,
specialty area of the laboratory contributes to the piec- from their observations, if the criminalist makes a con-
ing together of the puzzle—who did the crime, where did clusion in an area that may not be their expertise, the
they do it, and how did they do it. The criminalist may see conclusion may conflict with someone with expertise.
the same kinds of evidence day after day or never see Criminalists may make interpretations or conclusions,
the exact same thing twice. Sometimes he or she may but they must be certain to keep them separate from
be able to use what was learned from a previously ex- their observations and be within their area of exper-
amined case (sometimes from a similar incident many tise.
years before) to solve new problems.
A conclusion can be stated after an observation
While the crime scene is the place where the crime only when more information or data is developed
occurred, the crime laboratory is the place where evi- through further examination or testing that supports
dence reveals its secrets about the events of the crime. the conclusion. For example, if a red-brown colored
Using the tools of science, the criminalist answers ques- stain on a white T-shirt is spotted, he criminalist doesn’t
tions posed about the evidence, sometimes being asked assume it is blood. Dried blood can appear red to brown
to simply document what was found with no further or sometimes black in color. The stain could be rust, a
analysis. Other times, an identification and interpreta- dried tomato product such as catsup, spaghetti, or pizza
tion of the evidence is required. sauce, paint, ink, fingernail polish, or a variety of other
red-brown substances. Instead, he or she describes what
This is the first in a series of laboratory exercises, was observed in the working notes (a red-brown stain)
based on the book by John Houde, Crime Lab: A Guide and, where appropriate, document its location. If the
for Nonscientists, where you will explore the science stain has the color and texture of dried blood, the term
behind the criminalistics. Each exercise reflects real or “apparent blood,” or, “similar in appearance to blood”
simulated casework, and will give the student insight might be used. The stain requires more testing before a
into what it is like to work in a crime laboratory. conclusion can be made that the stain is blood. Even
after some preliminary chemical testing for blood one
Following are topics that generalize about the can only indicate that the red stain is “presumptively
criminalist’s typical duties in the laboratory. blood” or “consistent with blood” because other non-
blood substances, including some plants may react
Stating the Obvious positively with the presumptive blood test.

(and the not so obvious) Arthur Conan Doyle, through his character
Sherlock Holmes, provided great insight into poor hab-
Making observations and then stating the obvi- its of some crime scene investigators when he wrote,
ous (and sometimes stating the not-so-obvious), “It is a capital mistake to theorize before one has data.
whether in writing a report or in speech (testimony), Insensibly one begins to twist facts to suit theories in-
represents two very important characteristics of a crimi- stead of theories to suit facts.”
nalist. Investigators and attorneys rely on our observa-
tion skills and other tests of the physical evidence to Occasionally, while making observations in an
provide them with answers to questions about the case. attempt to answer the question or hypothesis posed by
The records of our observations can be very important an investigator or attorney, some other feature or at-
in helping other experts form their opinions. In a case tribute is observed by the curious criminalist which
where an automobile hits a pedestrian and then speeds leads to questions not originally asked. Curiosity is
off, being later found by investigator, the criminalist another important characteristic of a criminalist.
photographs, sketches, and describes all of the scrapes,
scratches, indentations, missing or broken parts of the Curiosity
vehicle. No conclusions are made about the observa-
tions. Later, an accident reconstruction expert may use Children, by nature, are inquisitive, and good
these observations to reconstruct the series of events criminalists learn to reawaken that childlike curiosity
surrounding the accident. and look at evidence with a new and fresh perspective.

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As adults, we have a great deal of life experience, a complete information and knowing what the conclu-
command of the language to translate those observa- sion should be, they figure that the information they
tions into words, and a purpose to our curiosity. Curi- have supports the conclusion. For instance, a red sub-
osity about some aspect of the evidence might provide stance, suspected of being very old blood, is found on
investigative leads, or help the legal system in its search a cloth. The faulty logic begins: the stain is red and blood
for the truth. Consider a case where the investigator is red, blood has hemoglobin and hemoglobin has iron
wants a criminalist to compare an evidence bullet in it, if I detect iron in the red portions of the cloth, the
against one fired from the suspect’s gun. The criminal- red substance must be blood. The scientist performs
ist notices a fiber and some white powder on the tip of the analysis and finds iron—there! It must be a blood
a bullet. A curious criminalist would document these stain. They reason that the stain is red, blood is red,
items and properly collect and preserve them, asking blood has iron in it, my tests show iron is present, there-
the investigator for more information about the events fore what I have on the cloth is blood. Unfortunately
of the crime. Perhaps the fiber simply came from the for this scientist, blood isn’t the only red substance that
victim’s clothing, or it may be that the bullet was fired has iron in it. For example, a painters’ red pigment, red
through a window (the white powder could be glass) ochre, is also red and contains iron. Further testing
and curtain. Perhaps the suspect never saw the victim. would be necessary before a conclusion that blood was
Criminalists specializing in trace evidence would care- present because finding iron alone doesn’t provide a
fully remove the powder and fiber and try to deter- conclusive identification for blood. The cautious foren-
mine their composition or source. sic scientist tries to not make “leaps of faith,” and if
they do, they must be ready to change their opinion if
A Leap of Faith and when new information or data requires it.

Sometimes a scientist will make a “leap of faith,” In a search for the truth, a theory must match all
or come to a conclusion or reach a theory before hav- of the evidence not some of the evidence. In order to be
ing enough information. Sometimes the leap leads to objective and fair, a criminalist must keep an open mind.
the correct conclusion and sometimes not. Having in- He or she may change a theory or interpretation as new
facts, information, or evidence is brought forth.

This unit was developed by Wayne Moorehead, MS, Fellow,
American Board of Criminalistics.

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Questions for Discussion:

1. How might a criminalist’s own natural curiosity affect his or her interpretations of evidence? In
a positive way? In a negative way?

2. In a closed room with two victims who were shot to death, how might the observation of tiny
specks of apparent blood around light bulbs and windows be interpreted?

3. How might pressure from attorneys and investigators to get lab results quickly influence the job
of a criminalist?

Copyright © 2002 Calico Press, LLC