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


Probable Event Analysis

The methods used to interpret evidence vary from investigator to investigator. Typically, an intuitive approach is used to interpret evidence, and the approach is not easily communicated from one investigator to the next. The only exception is the use of forensic evidence, which is scientifically based and limited in evidence interpretation. As a result, investigators are faced with abstract information derived from human observations and crime scene dynamics that must be woven together to generate an understanding of what may have occurred. The interpretation of evidence is currently a considerable weakness within the overall endeavor of criminal investigation. Evidence interpretation is not just a matter of understanding the meaning of evidence but also the identification of evidence. Understanding and identifying evidence are intertwined. Criminal defense investigation is riddled with this same weakness. A method founded in abductive reasoning is needed to eliminate the weakness of evidence interpretation as it relates to an alleged crime.

Methods of collecting evidence and determining where evidence may be found has been well defined in criminal investigations. However, the interpretation of evidence has been ill defined among law enforcement practitioners. Based upon the author’s own training, formal education, and experience as a working police officer, the interpretation of evidence by law enforcement officers is based upon dogmatic perceptions driven by egocentric bias. The opposing endeavor of criminal defense investigation is also riddled with bias, considering the foundation training of many criminal defense investigators is obtained in law enforcement or other bias-riddled professions. As a result, criminal investigations, by law enforcement or defense investigators, are impacted by the investigators’ own perceptions of evidence. The identification and interpretation of evidence related to an alleged crime is a subjective endeavor.

Two issues exist in relation to criminal evidence: the issue of measurement and the issue of credibility. Measurement is the examination of the diagnostic value of one piece of evidence in comparison to other evidence. On the other hand, credibility is the examination of the quality of specific evidence in respect of subterfuge or misconceptions. The most common themes of evidence credibility are misconceived comprehension and evidence trustworthiness. This is evident during a criminal trial, as the two opposing sides make arguments. These two themes can be generated by intentional or unintentional acts.

In theory, evidence of a crime is subjective, and is collected and interpreted based upon an investigator’s assumptions and investigative theory. In the end, the assumed evidence is further interpreted through logical arguments made by the prosecutor and defense attorney. This process of theory, assumption, collection, and interpretation starts with one single inquiry. An example is “Who killed Jane Doe?” In the same token, academic and professional researchers start a research endeavor with a single line of inquiry known as a research question. In essence, investigation and research may be considered the same form of inquiry. The only difference is the methods of inquiry used.

Based on the author’s own law enforcement experience, criminal investigations are based on intuitive reasoning, with some exceptions. In theory, criminal investigators employ logical deductive reasoning coupled with their expert opinion, in some cases. The expert opinion is usually derived from consultation with forensic scientists or the investigator's own training and professional experience. This is contrary to the conduct of the working professionals who actually pursue criminal investigations. In reality, criminal investigations can best be described as a fisherman casting a net. When the net is pulled in, fish caught in it is the prime suspect. From this point forward, all efforts are focused on this fish until a presumed logical argument is generated supporting the right to suspect the fish. This is the point where “evidence” truly becomes a phenomenon of perception.

Evidence as a phenomenon of perception derives from a wide array of causes, but in the case of criminal investigations, the overwhelming need is to bring closure. Typically, the vehicle for closure is the arrest and successful prosecution of a person. The byproduct of this need for closure is proclaiming the first fish is the correct one. In many cases, the guilt of the first fish is nothing more than a theory, but information uncovered throughout the investigation is subsequently judged based upon this theory. Information perceived to support the theory is deemed to be evidence, and all other information is rejected. This phenomenon is known as a “satisficing strategy.” A satisficing criminal investigation is one that is closed through an arrest and prosecution on the basis of one criminal theory.

From a criminal defense investigator’s perspective, evidence as a phenomenon of perception is a real and dangerous reality. In practice, a criminal defense investigator’s goal is to discredit existing evidence of guilt and provide new evidence to support an alternative theory.

Probable event analysis (PEA) is a structured analysis technique specifically used during criminal defense investigations. PEA is not meant to be used alone but as one of many possible instruments used in criminal defense. Most notably, PEA is an instrument of considerable utility in judging the credibility of evidence and discovering unknown evidence in a field investigation. The use of PEA is meant to be coupled with the method known as the analysis of competing hypotheses (ACH).
PEA produces logical arguments to discredit proposed evidence, assist the investigator in uncovering otherwise unseen evidence, and, most critically, enhance the development of alternate theories in relation to specific evidence at the “action level” versus the “person level.” PEA forces the criminal defense investigator to shift from the use of deductive perception to abductive perception. This provides a bridge from one criminal theory approach to an array of criminal theories.

Theoretical Lens of PEA

Through the theoretical lens of PEA, all events and specific items of evidence are viewed in opposing sets. In simple terms, no event or evidence supports a single theory or hypothesis but rather supports at least two opposing hypotheses. This theoretical lens is used to view the incident under inquiry through all segments of information associated with the incident. The overall incident is not considered as a whole, but the individual facets of the incident are assessed alone at the “action level.” Thus, two opposing hypotheses are proposed for every single segment of information.

The opposing hypotheses approach of PEA allows an investigator to assess an array of possibilities regarding what the evidence may represent. The utility of PEA is in its comprehensive approach to assessing the significance of evidence. This assessment is completed at the lowest level possible, the “action level.” For example, during a murder investigation, a single footprint is found in soft soil in the immediate area of the victim’s body. What does this footprint represent? Is it the murderer’s footstep? This possibly could lead to the identification of the murder, or at least, this is the view traditionally taken. On the other hand, is the footprint simply a footprint from an unrelated event? One hypothesis is used to examine the possibility that the footprint represents the footstep of the murderer, while the opposing hypothesis allows for another interpretation of the same evidence. What evidence exists to support one hypothesis versus another? Most evidence will support more than one hypothesis; however, the goal is to determine which evidence only supports one hypothesis. This evidence is core evidence.
The significance of some evidence, after careful consideration, has little to no value in determining historical events. Essentially, the evidence landscape presents a wide array of information for determining the historical events of an alleged crime. Many individual segments of information hold no utility in making an intelligent assessment of an historical event. The wide array of information available acts as a fog over the investigation, creating confusion. The key is determining which specific segments of information are connected to the alleged crime as evidence, known as critical evidence, and which segments represent irrelevant events in history.

In application, PEA is meant as an instrument of comparison. A single segment of information is assessed based upon the possibility that the information represents two opposing alternative views. In simple terms, a single segment of information is taken under consideration, and two possible explanations are explored. For example, a ball falls from the sky. Did the ball fall from outer space? Was the ball kicked from an adjacent area? Without any information other than the ball fell from the sky, both questions can be found to be true. However, more information is needed to fully assess both questions. The key is viewing each question as a scenario and then attempting to support each theoretical scenario with evidence. Was there a child in an adjacent area playing with a ball? Was a ball in orbit around the Earth? What other possible scenarios can be posed as questions and then answered through field investigation? The overall goal of PEA is sorting information through a comparative strategy of investigation.

In terms of criminal defense investigation, the comparative strategy should be highlighted by opposing, positive-negative perspectives. For every segment of information, one positive scenario, expressed through a hypothesis, and one negative scenario should be generated. In many cases, the negative scenario will already be proposed by law enforcement. In turn, the criminal defense investigator must generate an opposing alternative “positive” scenario. A positive scenario represents the innocence of the defendant or a scenario of an unrelated alternative event. However, when there are common items of physical evidence and common occurrences, the criminal defense investigator should assess multiple alternative positive scenarios.

The PEA is applied through the following steps:

  • Identify the information segment.
  • Determine the opposing positive-negative scenarios that the information segment possibly represents.
  • Search for information to support each positive and negative scenario.
  • Reassess to determine if any additional positive alternative scenarios exist.
  • Evaluate any information in support of opposing positive-negative scenarios.
  • If information exists supporting the positive and negative scenario, the information is accepted as neutral evidence.
  • If information exists supporting the positive scenario but not the negative scenario, the information is accepted as evidence of innocence.
  • If information exists supporting the negative scenario but not the positive scenario, the information is accepted as evidence of guilt.
  • If no information exists supporting the positive or negative scenario, the information is rejected as unrelated information.
The above five-step process should be repeated with every segment of information encountered by the criminal defense investigator. With each facet of information, two opposing scenarios are proposed as hypotheses: one negative and one positive. Then, additional information is searched for to support both hypotheses. Once this information is uncovered or confirmed to not exist, the segment of information under examination is evaluated based upon four possible outcomes. If information exists supporting the positive and negative scenarios, the information is accepted as neutral evidence.

The following is an example of an evaluation leading to the above result:

During a murder investigation, a single footprint is found in soft soil in the immediate area of the victim’s body.

  • Negative hypothesis: The defendant's shoes match the footprint found at the crime scene.
  • Positive hypothesis: The defendant’s shoes are issued by the largest area employer.
  • Information uncovered: The defendant owns a matching pair of shoes that were issued by the largest employer in the city. The number of employees issued identical shoes is approximately 3,256.
If information exists supporting the positive scenario but not the negative scenario, the information is accepted as evidence of innocence.
The following is an example of an evaluation leading to the above result: During a murder investigation, a single footprint is found in soft soil in the immediate area of the victim’s body.

  • Negative hypothesis: The defendant's shoes match the footprint found at the crime scene.
  • Positive hypothesis: The defendant’s shoes are issued by the largest area employer.
  • Information uncovered: The defendant does not own a matching pair of shoes that were issued by the largest employer in the city. The number of employees issued identical shoes is approximately 3,256.
If information exists supporting the negative scenario but not the positive scenario, the information is accepted as evidence of guilt.
The following is an example of an evaluation leading to the above result: During a murder investigation, a single footprint is found in soft soil in the immediate area of the victim’s body.

  • Negative hypothesis: The defendant's shoes match the footprint found at the crime scene.
  • Positive hypothesis: The defendant’s shoes are issued by the largest area employer.
  • Information uncovered: The defendant does own a matching pair of shoes. The largest employer in the city does not issue shoes to any employees.
If no information exists supporting the positive or negative scenario, the information is rejected as unrelated information.
The following is an example of an evaluation leading to the above result: During a murder investigation, a single footprint is found in soft soil in the immediate area of the victim’s body.

  • Negative hypothesis: The defendant's shoes match the footprint found at the crime scene.
  • Positive hypothesis: The defendant’s shoes are issued by the largest area employer.
  • Information uncovered: The defendant does not own a matching pair of shoes. The largest employer in the city does not issue shoes to any employees.
Understanding PEA in application is directly dependent on understating the concept of generating opposing hypotheses. The above example is overly simplistic and overly broad. However, it demonstrates how information is evaluated through PEA. In application, overly broad hypotheses would not be generated, for example, the idea of the defendant's shoe being issued by a large area employer. This example was only used to clearly communicate the concept of information evaluation.

The end goal of PEA is to enable the criminal defense investigator’s evaluation of information based upon the information's supporting information. Simply stated, is the information really evidence when considering its context from an alternative perspective? However, PEA is utilized in a field based environment. Therefore, the process is haphazard and should not be the primary instrument of diagnostic analysis during an investigation. Why even utilize the method considering the possibility of errors? Field operations allow the criminal defense investigator’s immersion in the environment where the crime allegedly occurred. This allows the criminal defense investigator to make considerations that are not adequately possible when outside of the environment. This environmental immersion provides insight, but it can also limit the criminal defense investigator's broader view. Thus, PEA is not utilized as a primary diagnostic instrument but rather as a diagnostic aid during collection operations.