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Law 101: Legal Guide for the Forensic Expert

Formulating a Working Hypothesis

After investigating and testing to establish the facts of a case, the expert should then formulate a working hypothesis. The following basic case example shows how this is done.

Example: The prosecution's hypothesis was that the defendant, charged with burglary, was:

  1. The perpetrator of the burglary.
  2. The seller of the stolen goods to the state's prime prosecution witness, who had purchased the recently stolen materials from the defendant.

The prosecution forensic fiber expert was able to analyze and conclusively establish fiber remnants from stolen materials in both the defendant's van and in his home closet.

The facts of the case emphasize the need for a working hypothesis in a forensic case, and application of that hypothesis to the discovered facts. Sometimes, the hypothesis is not easily determined, and considerable investigation and testing is required before a specific theory can be developed.

The expert should be prepared to review any evidentiary reports and confer with counsel about how the reports support an existing or tentative theory. The witness should be able to offer opinions and conclusions based on the reviewed reports. Such reports and report conclusions may elicit changes in either party's theory.

The expert should analyze the evidence and identify the positive evidence (matching profile) that cannot be excluded. Negative (nonmatching, excluded) or inconclusive evidence (i.e., a test was attempted but no results or uninterpretable data were obtained because of contamination, degradation, or an insufficient sample to test) should also be identified. The expert may need to explain the implications of these results.

The expert should be prepared to explain and identify all testing that was performed and to identify other tests that may not have been attempted (necessary for full disclosure and to rebut claims of the "CSI effect").

Some of the possible explanations the expert may need to include: 

  • The importance of negative evidence (either none exists, or no evidence was sent for testing).
  • Why there were no results (tests may have been attempted, but they were unusable or inconclusive, or no results were obtained).
  • Why having no conclusive results does not necessarily negate theories.
  • Why every item of evidence was not tested (prioritization, resources, etc.).

Various interpretations should be offered, and these should be consistent with the results. The attorney should be made aware of alternate theories that are supported by the results.

The attorney may need to be educated on the scientific terminology. Scientific terms should be explained to the attorney in appropriate language. The attorney must ensure that the descriptions and scientific terminology are accurate so that the jury is not misled.

The expert can expedite this procedure by preparing a glossary or information sheet specific to the case for the attorney to use on the witness stand. This glossary may also be useful to the court reporter at the deposition and trial.

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