Peters et al. Delusions Inventory (PDI-21)

The Peters et al.

Questions: 21
Duration: 10 min
Original title: PDI-21 — Peters et al. Delusions Inventory
Adaptation: Peters et al. Delusions Inventory (PDI-21)
Authors: Peters, E., Joseph, S., Day, S. & Garety, P.
Created: 2004

The Peters et al. Delusions Inventory (PDI-21) is a validated self-report instrument that captures the presence and subjective impact of delusional ideation. Rather than treating unusual beliefs as a simple binary phenomenon, the PDI-21 approaches them dimensionally. Clinicians rely on it to measure not only whether a specific belief is endorsed, but also how much distress it causes, how often it preoccupies the individual, and the degree of conviction with which it is held.

This multidimensional approach is crucial when evaluating everyday unusual experiences versus clinical psychotic symptoms. The PDI-21 helps mental health professionals chart out a patient's internal landscape, spanning from paranoia and grandiosity to somatic and religious themes. By separating the occurrence of a belief from its psychological burden, the instrument offers a nuanced metric that supports precise clinical formulations and guides targeted cognitive-behavioral interventions.

Example result

Fictitious result, computed from a sample set of answers.

Overall score

10/ 21

Self-report screening questionnaire for delusional ideation (psychotic ideation) in general and clinical populations, based on a dimensional model of psychoticism (normal/pathological continuum). It explores 21 unusual beliefs (ideas of reference, persecution, grandiosity, control, paranormal experiences, religiosity, guilt, thought process disorders, thought broadcasting/echo). For each item, the patient first indicates whether they have ever had this experience (Yes/No). In the event of an affirmative response, they then rate three dimensions associated with the belief: distress, preoccupation, and conviction, each scored on a 1 to 5 scale.

généralemixed · students · 17–30 years · Spain · n = 660 · 2012 · Fonseca-Pedrero et al. (2012)

021

M = 4.3 ± 2.78

généralemen · students · men · 17–30 years · Spain · n = 195 · 2012 · Fonseca-Pedrero et al. (2012)

021

M = 4.86 ± 3.17

généralewomen · students · women · 17–30 years · Spain · n = 465 · 2012 · Fonseca-Pedrero et al. (2012)

021

M = 4.06 ± 2.57

généralemixed · general population · healthy adults (validation sample, median 6) · United Kingdom · n = 444 · 2004 · Peters et al. (2004)

021

M = 6.7 ± 4.4

cliniquemixed · pathology: delusions · delusional patients (psychosis, median 11) · United Kingdom · n = 33 · 2004 · Peters et al. (2004)

021

M = 11.9 ± 6

Scoring

Number of beliefs endorsed (0 to 21), total score from 0 to 21, each item is scored 0 or 1.

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What the test measures

The PDI-21 evaluates the frequency and qualitative dimensions of delusional ideation across various thematic domains. It helps clarify if cognitive styles like jumping to conclusions solidify these unusual beliefs.

Occurrence: The raw number of unusual or delusional beliefs currently endorsed by the individual.
Distress: The amount of emotional pain or anxiety the endorsed belief generates.
Preoccupation: The frequency and intrusiveness of the thoughts related to the unusual belief.
Conviction: The absolute certainty or fixedness with which the individual holds the belief to be true.
Thematic Content: The specific nature of the beliefs, such as persecution, grandiosity, or magical thinking.

This instrument is designed for adults and can be used effectively across both clinical populations and the general population to gauge overall psychosis proneness.

When and why to use it

The PDI-21 is utilized when a clinician needs to quantify the subjective impact of delusional ideation rather than just cataloging odd thoughts.

Intake Baseline: To map the patient's internal reality and integrate it into a longitudinal case formulation.
Monitoring Change: To track fluctuations in conviction and distress over time, which helps evaluate coping strategies deployed in therapy.
Screening: To identify elevated risk for psychosis in individuals reporting distressing perceptual or cognitive anomalies.
Differential Support: To distinguish primary psychotic processes from the mood-congruent psychotic features sometimes observed alongside bipolar disorder warning signs.
Trait Assessment: To clarify whether beliefs reflect subclinical schizotypy often evaluated alongside personality disorders.

A validated score brings objective measurement to the clinical impression, allowing the clinician to separate the mere presence of a belief from the disabling distress it causes.

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The questions

We are interested in the beliefs and experiences that people may have. Many people have unusual experiences or beliefs. We would like to know if this type of experience has ever happened to you. Please answer YES or NO to each question, based on what you may have felt during your lifetime. There are no right or wrong answers; please answer as honestly as possible. If you answer YES to a question, you will then be asked to specify how much this experience preoccupied you, caused you distress, and how strongly you believe in it.
1. Do you sometimes feel that people are making hints about you or saying things with a double meaning?
Probes for ideas of reference in social interactions.
- No
- Yes
2. Do you sometimes feel that things in magazines or on TV were written especially for you?
Assesses referential thinking involving mass media.
- No
- Yes
3. Do you sometimes have the impression that some people are not what they seem?
Screens for interpersonal suspicion or persecutory hypervigilance.
- No
- Yes
4. Do you sometimes feel that you are being persecuted in some way?
Directly asks about subjective feelings of persecution.
- No
- Yes
5. Do you sometimes feel that there is a conspiracy against you?
Explores more organized, systemic paranoid ideation.
- No
- Yes
6. Do you sometimes feel that you are destined to be someone very important?
Evaluates grandiose beliefs regarding personal destiny.
- No
- Yes
7. Do you sometimes feel that you are a very special or unusual person?
Probes for grandiosity tied to identity and self-worth.
- No
- Yes
8. Do you sometimes feel that you are particularly close to God?
Assesses religious grandiosity or extreme spiritual connectedness.
- No
- Yes
9. Do you sometimes think that people can communicate telepathically?
Inquires about magical thinking and paranormal communication.
- No
- Yes
10. Do you sometimes feel that electrical devices like computers can influence the way you think?
Probes for delusions of influence or external control.
- No
- Yes
11. Do you sometimes feel that you have been chosen by God in some way?
Explores religious ideation involving divine selection.
- No
- Yes
12. Do you believe in the power of witchcraft, voodoo, or the occult?
Screens for beliefs in esoteric or occult forces.
- No
- Yes
13. Are you often worried that your partner might be unfaithful to you?
Assesses themes of morbid jealousy and relational paranoia.
- No
- Yes
14. Do you sometimes feel that you have sinned more than the average person?
Inquires about intense pathological guilt or moral failure.
- No
- Yes
15. Do you sometimes feel that people look at you strangely because of your appearance?
Examines referential anxiety centered on body image and social observation.
- No
- Yes
16. Do you sometimes feel like you have no thoughts in your head?
Probes for experiences of thought block or mental emptiness.
- No
- Yes
17. Do you sometimes feel that the world is about to end?
Assesses catastrophic, nihilistic, or apocalyptic ideation.
- No
- Yes
18. Do you sometimes feel that your thoughts are alien to you in some way?
Examines the attribution of heightened personal significance to cognitive events.
- No
- Yes
19. Do you sometimes feel that your thoughts are so vivid that you worry other people can hear them?
Screens for the subjective experience of thought broadcasting.
- No
- Yes
20. Do you sometimes feel that your thoughts are echoed back to you?
Inquires about the experience of thought echoing.
- No
- Yes
21. Do you sometimes feel like you are a robot or a zombie, with no will of your own?
Explores the belief that one's mind is fully transparent or being read.
- No
- Yes