Dissociation may be important across many mental health disorders, but has been variously conceptualised and measured. We introduced a conceptualisation of a common type of dissociative experience, ‘felt sense of anomaly’ (FSA), and developed a corresponding measure, the Černis Felt Sense of Anomaly (ČEFSA) scale.

We aimed to develop a short-form version of the ČEFSA that is valid for adolescent and adult respondents.

Data were collected from 1031 adult NHS patients with psychosis and 932 adult and 1233 adolescent non-clinical online survey respondents. Local structural equation modelling (LSEM) was used to establish measurement invariance of items across the age range. Ant colony optimisation (ACO) was used to produce a 14-item short-form measure. Finally, the expected test score function derived from item response theory modelling guided the establishment of interpretive scoring ranges.

LSEM indicated 25 items of the original 35-item ČEFSA were age invariant. They were also invariant across gender and clinical status. ACO of these items produced a 14-item short-form (ČEFSA-14) with excellent psychometric properties (CFI=0.992; TLI=0.987; RMSEA=0.034; SRMR=0.017; Cronbach’s alpha=0.92). Score ranges were established based on the expected test scores at approximately 0.7, 1.25 and 2.0 theta (equivalent to standard deviations above the mean). Scores of 29 and above may indicate elevated levels of FSA-dissociation.

The ČEFSA-14 is a psychometrically valid measure of FSA-dissociation for adolescents and adults. It can be used with clinical and non-clinical respondents. It could be used by clinicians as an initial tool to explore dissociation with their clients.