Luciano JV, Barrada JR, Aguado J, Osma J, García-Campayo J. Bifactor analysis and construct validity of the HADS: a cross-sectional and longitudinal study in fibromyalgia patients. Psychological Assessment. 2014 Jun;26(2):395-406.
McKenna SP, Doward LC. How should we measure quality of life in ankylosing spondylitis? Rheumatol Pract. 2008;6(2):12-4.