It has come to my attention that certain “camps” within the discussions surrounding the cloth analysis of the crime scene duct tape and the gas can duct tape are misrepresenting (either intentionally or out of ignorance – maybe wishful thinking or denial) statements made in the FBI documents. Statements are being made such as “they say it didn’t come from the same roll”, etc. This is incorrect, untrue, false – whatever classification you feel most comfortable assigning it. But, irrespective the label you choose – it’s not right. My personal opinion is that these statements are being made from a combination of bias (i.e. the person(s) do not want to accept the possibility the various pieces of duct tape are connected) and ignorance about the role of the FBI lab and the language used in their reports. So my hope in this entry is to clear some of that up.
ROLES
First let us address the role of the FBI laboratory reports. They are to present the actual test results, trained observations, etc. They are to deal in fact and to make a call on certain findings/observations from an analytical viewpoint. They do not interpret the data they present past the physical evidence they have, but they log their expert observations in a scientific method so that experts in the given field (DNA, fiber analysis, etc.) can use their observations with confidence to render an interpretation. In other words, the lab reports tend to be “just the facts, ma’am” without extraneous considerations added. When you read the reports you will see this with consistency. A good analogy of the roles of the laboratory personnel and the expert can be drawn by talking about having an X-ray, CT-scan or MRI. The laboratory technician performs the test, and can even record notes of observations they make during the test to pass onto the doctor who will be interpreting the results, but the technician does not render the interpretation. In fact, in most hospitals in the U.S. it is strictly against policy for a technician performing the test to make any statement of interpretation to the patient. The interpretation comes when the Radiologist (or whatever doctor is applicable for the type of test being performed) takes the resultant data (which can include the notes taken by the technician) and formulates an interpretation based on the data before him.
Another good example from the medical field is when you have blood work done. Let’s say the lab worker performs a full blood count and a differential blood count on your sample and returns the data of low reticulocytes, high leukocytes, and low neutrophils. The lab worker is not going to come find you and tell you it appears you have leukemia. The lab worker is going to produce a report to the oncologist who will then take into account all other testing performed, your symptoms, prior treatment, and any other extenuating circumstances to render that diagnosis, or some other. Why? Because even though all these lab results are included in the diagnosis of leukemia, having these lab results does not NECESSARILY mean you have leukemia. An expert must take into account ALL factors prior to rendering an expert interpretation.
This is equivalent to the role between the forensics lab and the expert who will ultimately review the findings and render an expert decision. Just as with the medical analogy, the expert will take into account more information than the laboratory personnel. For instance, in the case of the Radiologist, he will take into account the patient’s symptoms in interpreting the results. In the case of a forensics expert, he will take into account the extraneous factors that could play into effecting the observations made by the forensics lab.
Let’s apply these roles to the forensic results that have been published on the various pieces of duct tape and other evidence for which discovery has been released to date. The forensics lab’s job is to record all physical observations and to make a “call” on whether something is consistent, inconsistent, eliminated or unable to be eliminated (we’ll get to these terms in a bit). So they record everything physical they observe. In the case of fibers they identify the type of thread (cotton, wool, polyester, etc.), whether it is lusterous or delustered (i.e. has a shine or has been chemically treated in a manner that “de-shines” it), etc. In a comparative forensic test they will take a given fiber from a known source and they will compare to a fiber found on a second known source and they will make the call of “it’s like, it’s not like” and this call will have varying degrees of finality (the terms used above and discussed later). About the only time you will see a comment about extraneous factors (such as environment, etc.) in a forensics lab report is when those very factors prevent the lab employee from performing the analysis in the first place. For instance, in the case of Q104 (the last piece of Henkel duct tape found at the crime scene), the forensic lab worker states the weave cannot be analyzed due to degradation. Another place we see a note of extraneous factors is several of the pieces of plastic recovered from the crime scene where their state of degradation prevents forensic analysis. And then lastly, we see reference to the environment concerning the quality of DNA returns on the pieces of tape. After all observations/testing are finished, the lab’s final report is then passed to the chosen forensic expert in that area for interpretation.
TERMINOLOGY
The definition of some of the typical qualifiers to observations made in the lab – straight from the dictionary:
1 archaic : possessing firmness or coherence
2 a : marked by harmony, regularity, or steady continuity : free from variation or contradiction <a consistent style in painting> b : marked by agreement : compatible —usually used with with <statements not consistent with the truth> c : showing steady conformity to character, profession, belief, or custom <a consistent patriot>
3 : tending to be arbitrarily close to the true value of the parameter estimated as the sample becomes large <a consistent statistical estimator>
1 : having characteristics in common : strictly comparable
2 : alike in substance or essentials : corresponding <no two animal habitats are exactly similar — W. H. Dowdeswell>
3 : not differing in shape but only in size or position
1 : capable of or suitable for comparison
2 : similar, like <fabrics of comparable quality>
: lacking consistency: as a : not compatible with another fact or claim <inconsistent statements> b : containing incompatible elements <an inconsistent argument> c : incoherent or illogical in thought or actions : changeable d : not satisfiable by the same set of values for the unknowns <inconsistent equations> <inconsistent inequalities>
: unlike <people with dissimilar backgrounds>
transitive verb 1 a : to put an end to or get rid of : remove, eradicate <the need to eliminate poverty> <eliminate errors> b : to remove from consideration <cannot yet eliminate him as a suspect> c : to remove from further competition by defeating <the team was eliminated in the first round of the playoffs>
2 : to expel (as waste) from the living body
3 : to cause (as an unknown) to disappear by combining two or more mathematical equations
So in the laboratory reports that will be handed to the experts in this case, the following statements will be seen (“is not consistent with”, “is consistent with”, “cannot be eliminated as” and “can be eliminated as”. Forensics rarely have “is” and “is not” in its vocabulary. That is because forensics relies heavily on probability. The call will be based on “highly probable”, “probable”, “improbable” and “highly improbable”. There are occasions where we will see them outright declare an “is” or “is not” or “cannot”, but those instances are for a small set of tests with a small set of results. For instance, in parentage testing the call can be made definitively “…cannot be the father of…” That’s because it truly does come down to a black and white issue – does the proposed parent have the required alleles to produce the given offspring. Another type where a certainty can be issued is if the physical evidence just outright won’t “fit”. While ludicrous, the following statement illustrates what is meant by this “The 38 bullet that was retrieved from the victim’s heart did not issue from the suspect’s 22 caliber rifle.” Basically, when tab A is 3″ wide and slot B is .5″ wide – you can make a definitive call on the physicality of the evidence.
So focusing in on what the FBI reports state about the comparison of the crime scene duct tape and the gas can duct tape, we see that the language is as follows for the lab results of the analysis of the backing and adhesive (page 68 of this released discovery):
Based on the examinations conducted, the Q64 and Q66 pieces of duct tape are comparable to one another in all physical attributes and in the chemical composition of their backing and adhesive components. Therefore, they originated from the same source roll of tape or from rolls of tape manufactured in the same manner.
So the observations made by the lab employee are that the tape DID (they do not put any qualifier in this statement) come from either the same roll, or a roll made under the same manufacturing process.
Now let’s look at the FBI report concerning the comparison of cloth between crime scene and gas can duct tape (page 566 of this released discovery):
The fabric portion of specimens Q62 through Q64…and Q104…are microscopically dissimilar in fiber composition to the fiber composition of specimen Q66… Accordingly they are not consistent with originating from the same source as Q66.
Please note that unlike the definitive statement made in the first analysis, this statement is qualified. It does not state they did NOT come from the same roll. It does not state they COULD NOT come from the same roll. It states there is a difference (inconsistency) that could indicate they came from separate sources. This statement does NOT establish “the null hypothesis”.
THE NULL HYPOTHESIS
The null hypothesis, in the scientific method, is the antithesis of what has been hypothesized. If the null hypothesis is established, then the original hypothesis is flawed in some manner. So in our case the hypothesis is “the duct tape at the crime scene came from the same roll as the duct tape on the gas can”. The null hypothesis would be “the duct tape at the crime scene could not come from the same roll as the duct tape on the gas can”. When the null hypothesis cannot be established the situation reduces to the following statement “it cannot be eliminated”. In other words, since the null hypothesis stated in this paragraph has not been established for the various duct tape, the following statement must be made: “The possibility that the duct tape at the crime scene came from the same roll as the duct tape on the gas can CANNOT BE ELIMINATED.”
Why is this important? Because if the base data without additional factors cannot eliminate a possibility, then the expert can take that base data and with the combination of extenuating circumstances draw a reasonable, sound conclusion. The extenuating circumstances in this case are: Henkel’s production methods, environment in which the duct tape existed, and time in those environs.
I will close this with two examples that show you why the roles between lab and expert are important, and how the null hypothesis is critical to drawing proper forensic conclusions:
Scenario 1:
You work in a ballistics laboratory and you are sent a pistol, a spent casing and the slug from a spent bullet. You are asked to determine if the spent bullet was fired from the gun. You perform ballistic testing and you find that neither the barrel marks nor the firing pin pattern match the gun. You report that the markings on the bullet and casing “are not consistent with it being fired from this gun”. A firearms expert is called in. He analyzes the firearm and finds that it is as originally manufactured with no subsequent modifications. At trial, when he is asked “Could this bullet have been fired from this gun?” He states, “No, it could not have.” In this case, the null hypothesis was established – by the expert, not the ballistics tester.
Scenario 2:
Same scenario as above but this time you find that while the firing pattern on the casing does not match, the barrel markings do match the gun. The casing shows signs of a double strike on the firing end of the casing with shallower indentations than those made by the gun in ballistics testing. You report your findings to the expert stating “Due to the dissimilarities in the firing pin pattern, this bullet is not consistent with being fired from this gun.” The firearms expert takes over and finds that the firing mechanism on the gun has recently been replaced. After research he finds that the manufacture date of the new firing mechanism in the gun dates to around the same time as the murder. When asked in court “Could this bullet have been fired from this gun?” He would state, “Yes, it could have been.” Subsequent line of questioning:
“Why do you say that when the firing pattern does not match?”
“Because the firing mechanism appears to have been replaced subsequent to the murder. In addition, the double pattern on the firing end of the casing shows that the firing mechanism in the gun at the time of the murder was in a state of disrepair which could have resulted in the first attempt to fire the weapon failing.”
In this case, the original report from the ballistics lab pointed to the bullet not being fired from the gun, despite the barrel marking similarities. But once the expert brought in all factors associated with the evidence, it was found that the null hypothesis could not be established. The possibility of the gun firing the bullet remains as evidence before the jury.
Valhall.
Related posts:
- Timeline and Forensics Files Updated 10/29/09
- Timeline and Forensics Files updated 11/21/09
- Understanding Accumulated Degree Days
- Caylee Anthony case: Understanding the ‘dispute’ between Baez and Ashton
- Understanding Cell Phone ‘Pings’
Click here to join the discussion
