Confirmatory factor analysis and recommendations for improvement of the Autonomy-Preference-Index (API)

Validation of the German version of the Autonomy-Preference-Index (API), a measure of patients' preferences for decision making and information seeking.

Stepwise confirmatory factor analysis was conducted on a sample of patients (n = 1592) treated in primary care for depression (n = 186), surgical and internal medicine inpatients (n = 811) and patients with minor trauma treated in an emergency department (n = 595). An initial test of the model was done on calculation and validation halves of the sample. Both local and global indexes-of-fit suggested modifications to the scale. The scale was modified and re-tested in the calculation sample and confirmed in the validation sample. Subgroup analyses for age, gender and type of treatment setting were also performed.

The confirmatory analysis led to a modified version of the API with better local and global indexes-of-fit for samples of German-speaking patients. Two items of the sub-scale, 'preference for decision-making', and one item of the sub-scale, 'preference for information seeking', showed very low reliability scores and were deleted. Thus, several global indexes-of-fit clearly improved significantly. The modified scale was confirmed on the validation sample with acceptable to good indices of fit. Results of subgroup analyses indicated that no adaptations were necessary.

This first confirmatory analysis for a German-speaking population showed that the API was improved by the removal of several items. There were theoretically plausible explanations for this improvement suggesting that the modifications might also be appropriate in English and other language versions.