title: Structural validity of the tonic immobility scale in a population exposed to trauma: evidence from two large Brazilian samples.
abstract: BACKGROUND
  Tonic Immobility is a temporary state of motor inhibition in situations involving extreme fear. The first scale developed for its assessment was the 10-item Tonic Immobility Scale (TIS). However, there are still few studies on its structural (dimensional) validity. The objective of this study was to reassess the factor structure of the TIS applied to representative samples exposed to general trauma of two Brazilian mega-cities.
  
  
  METHODS
  The sample comprised 3,223 participants reporting at least one traumatic experience. In São Paulo (n = 2,148), a Confirmatory Factor Analysis (CFA) first tested the originally proposed two-dimensional structure. This was followed by sequential Exploratory Structural Equation Models to identify the best fitting model, and subsequently tested in Rio de Janeiro (n = 1,075) via CFA. Alternative reduced versions were further explored using the aggregate sample. Model-based Item Response Theory (IRT) location parameters were also investigated.
  
  
  RESULTS
  An absence of factor-based convergent and discriminant validity rejected the original proposition. However, the one-dimensional structure still held several residual correlations. Further exploration indicated the sustainability of reduced versions with seven (alternative A) and six (alternative B) items. Both presented excellent fit and no relevant residual item correlation. According to the IRT location parameters, items in alternative B covered a wider range of the latent trait. The Loevinger's H scalability coefficients underscored this pattern.
  
  
  CONCLUSIONS
  The original model did not hold. A one-factor solution was the most tenable in both large samples, but with significant item residual correlations, indicating that content redundancies persisted. Further reduced and simplified versions of the TIS proved promising. Although studies are yet to be carried out in other settings, it is the authors' impression that the restricted versions of the TIS are already apt for use in epidemiologic studies since the pros tend to outweigh the cons (as outlined in the Discussion section).
authors.affiliation: Department of Epidemiology, Institute of Social Medicine (IMS), State University of Rio de Janeiro (UERJ), Rio de Janeiro, RJ, Brazil.
authors.forename: Michael
authors.initials: M
authors.lastname: Reichenheim
authors.affiliation: Department of Psychology, Institute of Education, Federal Rural University of Rio de Janeiro (UFRRJ), Seropédica, RJ, Brazil.
authors.forename: Wanderson
authors.initials: W
authors.lastname: Souza
authors.affiliation: Department of Epidemiology, National School of Public Health (ENSP), FIOCRUZ, Rio de Janeiro, RJ, Brazil.
authors.forename: Evandro Silva Freire
authors.initials: ES
authors.lastname: Coutinho
authors.affiliation: Institute of Psychiatry (IPUB), Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro, RJ, Brazil.
authors.forename: Ivan
authors.initials: I
authors.lastname: Figueira
authors.affiliation: Department of Psychiatry, Federal University of São Paulo (UNIFESP), São Paulo, SP, Brazil.
authors.forename: Maria Inês
authors.initials: MI
authors.lastname: Quintana
authors.affiliation: Department of Psychiatry, Federal University of São Paulo (UNIFESP), São Paulo, SP, Brazil.
authors.forename: Marcelo Feijó
authors.initials: MF
authors.lastname: de Mello
authors.affiliation: Department of Psychiatry, Federal University of São Paulo (UNIFESP), São Paulo, SP, Brazil.
authors.forename: Rodrigo Affonseca
authors.initials: RA
authors.lastname: Bressan
authors.affiliation: Department of Psychiatry, Federal University of São Paulo (UNIFESP), São Paulo, SP, Brazil.
authors.forename: Jair
authors.initials: J
authors.lastname: de Jesus Mari
authors.affiliation: Department of Psychiatry, Federal University of São Paulo (UNIFESP), São Paulo, SP, Brazil.
authors.forename: Sergio Baxter
authors.initials: SB
authors.lastname: Andreoli
mesh_terms: D000328:Adult; D001938:Brazil; D002947:Cities; D005163:Factor Analysis, Statistical; D005239:Fear / Q000523:psychology; D005260:Female; D006801:Humans; D046353:Immobility Response, Tonic; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D011594:Psychometrics; D013315:Stress, Psychological / Q000523:psychology*
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