A randomised comparison of a four- and a five-point scale version of the Norwegian Function Assessment Scale

Background: There is variation in the number of response alternatives used within health-related questionnaires. This study compared a four- and a five- point scale version of the Norwegian Function Assessment Scale (NFAS) by evaluating data quality, internal consistency and validity. Methods: All inhabitants in seven birth cohorts in the Ullensaker municipality of Norway were approached by means of a postal questionnaire. The NFAS was included as part of The Ullensaker Study 2004. The instrument comprises 39 items derived from the activities/participation component in the International Classification for Functioning, Disabilities and Health (ICF). The sample was computer-randomised to either the four- point or the five- point scale version. Results: Both versions of the NFAS had acceptable response rates and good data quality and internal consistency. The five- point scale version had better data quality in terms of missing data, end effects at the item and scale level, as well as higher levels of internal consistency. Construct validity was acceptable for both versions, demonstrated by correlations with instruments assessing similar aspects of health and comparisons with groups of individuals known to differ in their functioning according to existing evidence. Conclusion: Data quality, internal consistency and discriminative validity suggest that the five-point scale version should be used in future applications.