ANALYSIS OF EVIDENCE: SOME PRELIMINARY CONSIDERATIONS
Science is a way of examining the world and learning about it. The process of science, the scientific method, is proposing and refining of plausible explanations about any unknown situation. It involves asking and answering questions in a formal way and then drawing conclusions from the answers. Science, through its method, has two hallmarks. The first is the questions that are asked must be testable (or have testability). Asking “How many angels can dance on the head of a pin?” or “Why do ghosts haunt this house?” is not scientific because a test cannot be constructed to answer either of these questions. The second hallmark of science is repeatability. Science is a public endeavor, and its results are published for many reasons, the most important of which is for other scientists to review the work and determine whether it is sound. If nobody but you can make a particular experiment work, it isn’t science. Other scientists must be able to take the same kinds of samples and methods, repeat your experiments, and get the same results for it to be science (see “History: The Method of Science” for a discussion of scientific models).
In the language of science, the particular questions to be tested are called hypotheses. Suppose fibers are found on the bed where a victim has been sexually assaulted. Are the fibers those of the victim, the suspect, or someone else? The hypothesis could be framed as follows: “There is a significant difference between the questioned fibers and the known fibers from the suspect’s clothes.” Notice that the hypothesis is formed as a neutral statement that can be either proven or disproven. After the hypothesis has been formed, the forensic scientist seeks to collect data that shed light on the hypothesis. Known fibers from the suspect are compared with those from the scene and the victim. All relevant data will be collected without regard to whether it favors the hypothesis. Once collected, the data will be carefully exam ined to determine what value they have in proving or disproving the hypothesis; this is the probative value of the data. If the questioned fibers are analytically indistin guishable from the known fibers, then the hypothesis is rejected. The scientist could then conclude that the questioned fibers could have come from the suspect. But suppose that most of the data suggest that the suspect is the one who left the fibers there, but there are not enough data to associate the fibers to that source. It cannot be said that the hypothesis has been disproved (there are some similarities), but neither can it be said that it has been proved (some differences exist, but are they significant?). Although a scientist would like to be able to prove unequivocally that the garment is or is not the source of evidence, doing so is not always possible. As previously mentioned not all evidence can be individualized. The important point to note here is that evidence analysis proceeds by forming many hypotheses and perhaps rejecting some as the investigation progresses. Some preliminary questions must be answered before we even begin to formulate hypotheses. Is there sufficient material to analyze? If the amount of evidence is lim ited, then choices have to be made about which tests to perform and in what order. The general rule is to perform nondestructive tests first because they conserve mate rial. Most jurisdictions also have evidentiary rules that require that some evidence be kept for additional analyzes by opposing experts; if the entire sample will be consumed in an analysis, then both sides must be informed that not enough evidence will be available to have additional analyzes performed. If extremely large amounts of material are submitted as evidence, how are they sampled? This situation often happens in drug cases in which, for example, a 50-pound block of marijuana or several kilograms of cocaine are received in one package. The laboratory must have a protocol for sampling large quantities of mate rial so that samples taken are representative of the whole. In other kinds of cases in which this situation occurs, many exhibits may appear to contain the same thing, for example, 100 0.5-ounce packets of white powder. The laboratory and the scientist must decide how many samples to take and what tests to perform. This decision is especially important because the results of the analyzes will ascribe the characteristics of the samples to the whole exhibit, such as identifying 1000 packets of powder as 23% cocaine based on analysis of a fraction of the packets. What happens in cases in which more than one kind of analysis must be done on the same item of evidence? Consider a handgun received into evidence from a
shooting incident; it has red stains and possible fingerprints on it. This means that firearms testing, serology, latent print, and possibly DNA analysis must be performed on the handgun. These analyzes should be put into an order such that one exam does not spoil or preclude the subsequent exam(s). In this case, the order should be first serology, then latent print, and finally firearms testing. It is important to note that one seemingly small piece of evidence can be subjected to many examinations. Consider the example of a threatening letter, as depicted in Figure 3.6, one that supposedly contains anthrax or some other contagion. The envelope and the letter could be subjected to the following exams, in order:
- Disease diagnosis, to determine if it really contains the suspected contagion;
- Trace evidence, for hairs or fibers in the envelope or stuck to the adhesives (stamp, closure, tape used to seal it);
- DNA, from saliva on the stamp or the envelope closure;
- Questioned documents, for the paper, lettering and other aspects of the form of the letter;
- Ink analysis, to determine what was used to write the message, address, etc.;
- Handwriting, typewriter, or printer analysis, as appropriate;
- Latent fingerprints;
- Content analysis, to evaluate the nature of the writer’s intent and other investigative clues.
In this example, the ordering of the exams is crucial to ensure not only the integrity of the evidence, but also the safety of the scientists and their co-workers. Other evidence can also be very, very large—ocean currents, for example (see “In More Detail: Rubber Duckies and Human Remains”). It is important to realize that anything can become evidence and forensic scientists must keep open minds if they are to solve the most difficult of crimes.
FIGURE 3.6 Even one small item of evidence can be subjected to multiple examinations and may travel through most of a forensic laboratory. A threat letter, like this one, could pass through bacterial diagnosis, trace evidence, DNA, questioned documents, latent print analysis and content analysis.
