Influence of subjective factors on the formation of probabilistic conclusions in forensic handwriting examinations

Abstract

The most important step in conducting a handwriting examination is the process of forming conclusions. An expert has a sound basis to draw conclusions only when qualitative and quantitative assessments of all identified features clearly demonstrate the presence or absence of identity.

By the nature of expert tasks, conclusions are classified into diagnostic, classification, identification, and situational:

• Diagnostic conclusions provide information on the presence or absence of unusual writing conditions of a temporary or permanent nature:

  1. Internal factors: stress, illness, alcohol intoxication, age-related changes;

  2. External/situational factors: unusual posture, writing surface, or writing instrument, writing without prescription glasses, cold environmental conditions;

  3. Artificial factors: imitation of hand-printed letters, copybook script, another person's handwriting/signature, or deliberate handwriting modification (e.g., using the non-dominant hand).

• Classification conclusions reflect the object's affiliation with a specific class, such as identifying gender, age, handwriting type, or establishing general handwriting similarity (where individual identification is not performed).

• Identification studies are characterized by two types of findings: individual identity or group affiliation (established when individual identity cannot be proven).

In addition, expert conclusions are categorized by the degree of certainty into categorical (definitive) and probabilistic (likely):

• Categorical positive conclusions establish a specific individual as the author or executor of the questioned document.

• Categorical negative conclusions exclude an individual based on stable, significant differences in general and specific handwriting features.

Thus, an expert's conclusion is definitive (positive or negative) only when the investigation completely rules out the possibility of the opposite assertion.

A probabilistic conclusion indicates two key facts:

1. Scientific examination did not reveal features that would prevent a positive resolution of the issue;

2. The findings support an assumption, but are insufficient for a categorical conclusion.

The basis for distinguishing between categorical and probabilistic conclusions lies in the degree of certainty regarding the truth of the knowledge and the level of scientific validity supporting the finding.

Success in this process heavily relies on subjective factors, such as the expert’s personal experience, qualifications, and specialization. However, relying solely on subjective criteria when evaluating identified features can lead to erroneous conclusions.

Analysis of forensic practice shows that most errors in handwriting identification stem from the inability to properly identify, analyze, and evaluate handwriting features:

• Some experts base their findings on common features frequently observed in the handwriting of different individuals, which lack distinct identification value.

• An objective evaluation of identification features relies on their frequency of occurrence across different writers. By applying statistical probability methods, an expert can evaluate both the quantitative frequency and the qualitative significance of specific handwriting features.

Furthermore, the overall success and capability of resolving questions posed by investigative and judicial authorities depend on the following factors:

1. The type, nature, and specific characteristics of the questioned handwriting objects;

2. The availability of a sufficient quantity and quality of comparative samples;

3. The maturity and design level of the scientific methodology applied;

4. The technical equipment and analytical capabilities available to the expert laboratory.