Objectives: This study aimed (i) to determine the factor structure of the 12-item General Health Questionnaire (GHQ-12) across the cancer trajectory represented by samples from three cancer care settings and (ii) to appraise the item misfit and differential item functioning (DIF) of the GHQ-12.
Data and methods: Data were from cancer outpatient (n = 200), general community (n = 364) and palliative care (n = 150) settings. The factor structure was tested using exploratory factor analysis followed by confirmatory factor analysis. The factors were assessed for correlation using Spearman's ρ. The analyses were run separately for standard GHQ, Likert, modified Likert and chronic GHQ scoring and for the individual cancer settings. The best scoring method within the cancer setting was determined by Akaike's information criterion (AIC). Item misfit (mean square, MNSQ; standardised z-score, ZSTD) and DIF were assessed using the Rasch model.
Results: The best scoring method was the chronic GHQ for the cancer outpatient (AIC = -45.8), modified Likert for the general community (AIC = 9.6) and standard GHQ for the palliative care (AIC = -43.0). The GHQ-12 displayed a correlated two-factor structure ('social dysfunction' and 'distress'); Spearman ρ values were 0.69, 0.82 and 0.88 in the cancer outpatient, the general community and the palliative care, respectively. One item in the palliative care indicated misfit (MNSQ = 1.62, ZSTD = 3.0). Five items in the cancer outpatient showed DIF by gender and age. Two items in the palliative care showed DIF by gender.
Conclusions: The GHQ-12 was more problematic (less clear factor structure and evidence of item bias) for newly diagnosed patients, less problematic for patients approaching end-of-life and satisfactory for patients between those times.
Copyright © 2011 John Wiley & Sons, Ltd.