Criminal Defense Investigation: Theory, Practice, and Methods

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Recommended citation:

Pennington, Jeremy. Criminal Defense Investigation: Theory, Practice, and Methods. Ironton: Pennington & Associates Ltd., 2016


Key Assumption Check

During any investigation or general analysis, we subconsciously or intentionally make what are referred to as key assumptions. Recognizing that we make assumptions is a critical factor during any investigation. If we do not recognize that assumptions are being made, we stand a chance of having a critical failure of the overall investigation. A key assumption check should be completed at the onset of any criminal defense investigation and on a recurring basis throughout the investigation. Moreover, the criminal defense investigator should determine any assumptions made by the State during the initial stages of an investigation.

Defining a Key Assumption

A key assumption is an assumption that has a critical impact on the investigation. If a key assumption is incorrect, the whole investigation will collapse. For example, a common key assumption made by investigators is the credibility of a witness. We assume the witness has good eyesight, hearing, and the ability to detect odors. If this key assumption is incorrect, the witness’s testimony should be discarded. If we hold to the key assumption the witness is able to make an observation and provide credible testimony and this key assumption is incorrect, the resulting investigative conclusion will be erroneous.

A key assumption can be made in many forms, from taking a witness’s testimony at face value to assumptions that a human being is able to perform a specific physical action. In simple terms, key assumptions made during an investigation can create a critical investigative failure. All key assumptions made during an investigation must be articulated in a formal, structured process. This process is referred to as a key assumption check.

The first and critical step of completing a key assumption check is admitting that an assumption is being made. In practice, this requires a criminal defense investigator to exercise a high degree of self-discipline. All aspects of the investigation must be objectively reviewed and assessed for assumptions.

In addition to key assumption, key unknowns exist within a case. In simple terms, a key unknown refers to information that is unknown or denied. In theory, a key unknown is the opposing theoretical view of a key assumption. A key unknown is comprised of information that is truly unknown for any number of reasons. This information may be at the time denied by the State or unattainable by the criminal defense investigator. In either case, the information is simply unknown. A key unknown is not silent evidence; it is information known to exist that is unattainable. The criminal defense investigator must account for this unknown information recognizing the key unknown's potential to force a change in the investigation at a later junction or cause a critical failure.

Like a key assumption, the existence of a key unknown is detrimental to the overall analysis work conducted by a criminal defense investigator. If a key unknown is ignored during the investigation, the whole investigation is subject to the risk of collapse. Discerning the difference between a key assumption and key unknown can be confusing. Fully understanding the difference is critical during the application of a key assumption check. A key assumption is a true assumption made by the criminal defense investigator that is logically sound. On the other hand, a key unknown is specific information that is completely unknown to the criminal defense investigator and no reasonable assumption can be made to fill the information gap. Thus, the key unknown has a higher probability of causing an investigative failure. Ideally, a key unknown should be eliminated through further investigation, but this may not always be possible.

The Key Assumption Check Method

A key assumption check is completed through the following steps:

  • Determine all current assumptions being made during the investigation by challenging all investigative interpretations of alleged events.
  • Is there evidence to support each interpretation?
  • Has each interpretation been challenged through field investigation?
  • Is there an opposing interpretation by another investigator or interested party?
  • Is it possible to hold an opposing interpretation?
  • Does corroborating evidence exists to support the interpretation?
  • After identifying all assumptions, assess each assumption through the following questions:
  • Why is the assumption being made?
  • In what circumstances could the assumption be wrong?
  • What is the confidence level of the assumption?
  • Could any emerging evidence impact the assumption and make the assumption invalid?
  • If the assumption turns out to be invalid, what impact will this have on investigative conclusions?
Organize the assumptions into the following three categories:

  • Reasonably solid
  • Correct with some caveats
  • Unsupported—the “key unknowns”
  • Convert any key unknowns into investigative priorities.

Key Assumption Check in Practice

The key assumption check brings an important question to the table, relevant to both for-profit or public investigations: Why would an investigator make assumptions? Theoretically, an investigator should never make an assumption. Many professional and public investigators will argue their work product does not contain any assumptions; however, upon close examination, this is simply not true. Reflecting on the previous example, how many investigators confirm an eyewitness is capable of making the alleged observation? This form of confirmation is rare in professional and public investigations. Why would an assumption be made about an eyewitness’s ability to observe an incident? In most circumstances, this form of assumption is based upon solid logical reasoning. An eyewitness whose observations are made during daylight and in close proximity is an example of a logical, sound assumption of the witness’s ability to make those observations. On the other hand, if the witness had made the same observations during low-light conditions from a greater distance than the same assumption would be considerably less sound. The first scenario presents an assumption categorized as “correct with some caveats.” The caveat is the possibility that the witness is dependent on eyeglasses, which may have or may not have been worn during the observations. The second scenario differs considerably from the first. In this scenario, the witness is making the same observations during low-light conditions at a greater distance. The assumption in this scenario would be categorized as “unsupported.” There are just to many unanswered questions to make a logically sound assumption regarding the witnesses capability of making those observations. The key question in both scenarios is whether the witness”s observations are detrimental to the investigation.

Analysis in a criminal defense investigation requires a micro-level approach to information. This micro approach is a critical aspect in applying a key assumption check. Overly broad assumptions are highly problematic and carry an extreme liability of failure. Continuing the witness example above, are the witness’s observations detrimental to the investigation? This is an important question, and the answer can be used to differentiate between an assumption and a key assumption. However, a simplistic example will not demonstrate how to make this judgement. The assumption has to be weighed against the entire alleged incident. What do the witness's observations prove? What impact does this proof have on the overall alleged criminal act? The impact of any evidence and its underlining assumptions has to be taken into account using a holistic approach.

Every criminal defense investigation will be comprised of at least some key assumptions. The goal is to determine which assumptions are key in the investigation and to assess the underlining evidence supporting those assumptions. The identification and evaluation of key assumptions is critical to prevent possible failure. Moreover, the identification of key unknowns through a key assumption check allows the investigator to close investigative gaps that are detrimental to the overall investigation.