Exploratory Factor Analysis of SCL90-R Symptoms Relevant to Psychosis
Bakhshaie, Jafar, Sharifi, Vandad, Amini, Javad, Iranian Journal of Psychiatry
Objective: Inconsistent results have been reported regarding the symptom dimensions relevant to psychosis in symptoms check list revised (SCL90-R), i.e., "psychoticism" and "paranoid ideation". Therefore, some studies have suggested different factor structures for questions of these two dimensions, and proposed two newly defined dimensions of "schizotypal signs" and "schizophrenia nuclear symptoms". We conducted an exploratory factor analysis on the items of these two dimensions in a general population sample in Iran.
Method: A total of 2158 subjects residing in Southern Tehran (capital of Iran) were interviewed using the psychoticism and paranoid ideation questions in SCL90-R to assess severity of these symptom dimensions. Factor analysis was done through SAS 9.1.3 PROC FACTOR using Promax rotation (power=3) on the matrix of "polychoric correlations among variables" as the input data.
Results: Two factors were retained by the proportion criterion. Considering loadings >= 0.5 as minimum criteria for factor loadings, 7 out of 10 questions from psychoticism ,and 3 out of 6 questions from paranoid ideation were retained, and others were eliminated. The factor labels proposed by the questionnaire suited the extracted factors and were retained. Internal consistency for each of the dimensions was acceptable (Cronbach's alpha 0.7 and 0.74 for paranoid ideation and psychoticism respectively). Composite scores showed a half-normal distribution for both dimensions which is predictable for instruments that detect psychotic symptoms.
Conclusion: Results were in contrast with similar studies, and questioned them by suggesting a different factor structure obtained from a statistically large population. The population in a developing nation (Iran) in this study and the socio-cultural differences in developed settings are the potential sources for discrepancies between this analysis and previous reports.
Keywords: Mental health, Paranoid disorders, Psychological Tests, Psychotic symptoms, Statistical factor analysis
Iran J Psychiatry 2011; 6:128-132
SCL90-R is a well known self report instrument to assess the psychological symptom status of individuals from "healthy controls" to "disordered ones" (1). It consists of 90 questions defined in 9 symptoms dimensions (depression, anxiety, phobic anxiety, hostility, obsessive-compulsive, interpersonal sensitivity, somatization, paranoid ideation and psychoticism dimensions). A good level of consistency and test-retest reliability has been reported for SCL90-R (2, 3).
The application of the instrument, with specific focus on two dimensions of psychosis (paranoid ideation and psychoticism dimension), has been done in at least two different models.
One approach used these two dimensions to detect psychotic like experiences in the general population (4, 5, 6). In fact, the researchers have used either the sum or mean of scores on the two dimensions as the reference score, and named the subjects with scores
above the 75 to 90 percentiles "positive" for psychotic like experiences and predisposed to psychosis. In other studies, the percentage of affirmative answers has been used to detect the rate of psychotic symptoms in the general population based on an accepted cut off for the distressfulness of symptoms mentioned in the two dimensions (7, 8, 9).
Regarding the uncertain validity of the SCL90-R psychosis symptom dimensions which has been replicated in some studies and has raised the need for reanalysis (7, 10), it should be noted that Rössler and colleagues , in their twenty- year prospective study named "Zurich study", have also conducted an exploratory factor analysis on the questions of these two dimensions to determine classes of psychotic experiences in them (9). Interestingly, they extracted two new dimensions of "schizotypal signs" and "schizophrenia nuclear symptoms "out of the original ones, by changing the order of some questions and omitting some others through their longitudinal six-step interview analysis. …