Investigation of In-Session Delusion Formation Scale (IIDF-25)

Description

The Investigation of In-Session Delusion Formation Scale (IIDF-25) is a specialized psychometric instrument developed to systematically assess the psychotherapeutic processes associated with the emergence and development of delusional ideas during psychotherapy sessions. The scale was developed by Zgantzouri and Nestoros (1999) as part of psychotherapy process research focusing on individuals with paranoid schizophrenia and schizoaffective disorder, with the aim of investigating the psychological mechanisms involved in the formation of delusional beliefs.

The IIDF-25 consists of 25 items designed to evaluate therapeutic interactions, cognitive processes, emotional experiences, and in-session events that may contribute to the development, activation, or maintenance of delusional ideation. Unlike conventional symptom rating scales, the IIDF-25 emphasizes the dynamic psychotherapeutic process, examining how delusional ideas emerge and evolve within the therapeutic encounter.

Responses are rated using a 7-point Likert scale, ranging from “Strongly Disagree” to “Strongly Agree”, allowing trained external evaluators to assess the extent to which specific mechanisms of delusion formation are observed during individual psychotherapy sessions.

The instrument is intended to be completed individually for each patient, taking into consideration the unique characteristics of each psychotherapy session. Because delusion formation is considered a dynamic and context-dependent phenomenon, interpretation is based on multiple therapeutic observations rather than isolated clinical events.

The IIDF-25 is primarily applied in:

  • Clinical Psychology
  • Psychotherapy of Psychotic Disorders
  • Psychiatry
  • Psychopathology
  • Psychotherapy Process Research
  • Schizophrenia Research
  • Clinical Outcome Research
  • Academic Research in Severe Mental Disorders

Assessment using the IIDF-25 contributes to a deeper understanding of the psychological mechanisms underlying delusion formation and provides valuable information for evaluating therapeutic processes and refining psychotherapy approaches for individuals with psychotic disorders.

Objective

The primary objective of the IIDF-25 is to provide a systematic and reliable assessment of the psychotherapeutic mechanisms associated with the emergence and development of delusional ideas during psychotherapy sessions.

More specifically, the instrument aims to:

  • Assess in-session mechanisms involved in the formation of delusional beliefs.
  • Examine the cognitive and emotional processes accompanying delusional ideation.
  • Evaluate therapeutic interactions that may facilitate or modify delusional thinking.
  • Improve understanding of the psychopathological processes underlying schizophrenia and schizoaffective disorder.
  • Support the evaluation of psychotherapeutic interventions for psychotic disorders.
  • Facilitate psychotherapy process research.
  • Contribute to the development of more effective psychotherapeutic approaches for individuals experiencing psychosis.

The IIDF-25 is designed exclusively for psychotherapy process evaluation and is completed separately for each individual patient based on observations made during therapeutic sessions.

Scoring

The IIDF-25 consists of 25 items, each rated on a 7-point Likert scale, ranging from “Strongly Disagree” to “Strongly Agree.”

For each proposed mechanism of delusion formation, an overall score is calculated as the mean value of all items representing that mechanism.

Higher scores indicate greater agreement by the external evaluator that a particular mechanism of delusion formation was evident during the psychotherapy session, whereas lower scores indicate limited or absent evidence for that mechanism.

Because the instrument evaluates psychotherapy processes rather than stable personality traits, interpretation should always be based on the broader therapeutic context, including multiple therapy sessions, clinical history, and comprehensive psychological assessment. The IIDF-25 is intended as a psychotherapy process evaluation instrument rather than a diagnostic tool.

Data Analysis and Applications

Statistical analysis of IIDF-25 data includes descriptive, psychometric, and inferential statistical procedures designed to evaluate both the psychometric properties of the instrument and the therapeutic mechanisms associated with delusion formation.

Descriptive Statistics

Initial analyses typically include:

  • Mean
  • Median
  • Standard Deviation
  • Minimum and Maximum values
  • Range
  • Frequencies and Percentages

These analyses provide an overview of the occurrence and intensity of delusion-formation mechanisms across psychotherapy sessions.

Reliability Analysis

Psychometric reliability may be evaluated using:

  • Cronbach’s Alpha
  • McDonald’s Omega
  • Corrected Item–Total Correlations
  • Split-Half Reliability
  • Test–Retest Reliability, where repeated observations are available.

According to the original validation study, the instrument demonstrated good internal consistency and satisfactory item homogeneity. Because the measured processes vary across psychotherapy sessions, more stable estimates are obtained by averaging observations from multiple sessions rather than relying on a single therapeutic encounter.

Validity Assessment

The available evidence indicates that the IIDF-25 demonstrates:

  • Face Validity
  • Content Validity

At the time of its development, criterion validity could not be established because no comparable instruments assessing the same psychotherapy process were available.

Future psychometric investigations may further evaluate the instrument through:

  • Exploratory Factor Analysis (EFA)
  • Confirmatory Factor Analysis (CFA)
  • Convergent Validity
  • Discriminant Validity

Group Comparisons

The IIDF-25 may be used to compare:

  • Different diagnostic groups
  • Different psychotherapy approaches
  • Successive psychotherapy sessions
  • Various stages of treatment

using statistical procedures such as:

  • Independent Samples t-test
  • Paired Samples t-test
  • Analysis of Variance (ANOVA)
  • Mann–Whitney U Test
  • Kruskal–Wallis Test
  • Analysis of Covariance (ANCOVA)

Correlation Analysis

IIDF-25 scores may be examined in relation to variables such as:

  • Delusional severity
  • Positive psychotic symptoms
  • Therapeutic alliance
  • Overall functioning
  • Symptom severity
  • Cognitive functioning
  • Quality of life
  • Treatment outcomes

Relationships are typically evaluated using Pearson or Spearman correlation coefficients, depending on the characteristics of the data.

Predictive Modeling

Advanced statistical procedures commonly include:

  • Simple Linear Regression
  • Multiple Linear Regression
  • Multilevel (Hierarchical) Modeling, particularly when multiple psychotherapy sessions are analyzed for each patient
  • Structural Equation Modeling (SEM)

These analyses facilitate the investigation of therapeutic mechanisms contributing to delusion formation and provide valuable insight into psychotherapy process research involving psychotic disorders.

References

  • American Psychiatric Association. (2022). Diagnostic and Statistical Manual of Mental Disorders (5th ed., Text Revision; DSM-5-TR). American Psychiatric Publishing.
  • Freeman, D. (2016). Persecutory Delusions: A Cognitive Perspective on Understanding and Treatment. The Lancet Psychiatry, 3(7), 685–692.
  • Garety, P. A., Kuipers, E., Fowler, D., Freeman, D., & Bebbington, P. E. (2001). A Cognitive Model of the Positive Symptoms of Psychosis. Psychological Medicine, 31(2), 189–195.
  • Zgantzouri, K. (2000). Psychotherapeutic Processes in Psychoses: Delusion Formation in Paranoid Schizophrenia and Schizoaffective Disorder (Doctoral Dissertation). University of Crete, Department of Psychology.
  • Zgantzouri, K. A., & Nestoros, J. N. (2000). Psychotherapy Process Research in Schizophrenia Paranoid Type: The Investigation of Delusion Formation Through the Evaluation of In-Session Events. Proceedings of the 13th International Symposium for the Psychological Treatments of Schizophrenia and Other Psychoses, Stavanger, Norway.