Figure 1 Scree plot of the eigenvalues (y axis) for factors (x axis) identified by item level exploratory principal component factor analysis of the 39 item Parkinson's disease questionnaire (PDQ‐39) and 1000 parallel sets of randomly generated PDQ‐39 data. Plots represent PDQ‐39 eigenvalues (empirical data) and the 95th percentiles of 1000 random data eigenvalues. The broken horizontal line indicates the cut off point for determination of the number of factors (scales) according to the eigenvalue >1 criterion.⁷ This criterion identified eight factors (Kaiser–Meyer–Olkin measure of sampling adequacy 0.92; Bartlett's test of sphericity χ², 4390.2, p<0.0001), of which the first four were stronger than those produced by random data. Contents of these four factors are indicated. The first four empirical and random factors explained 59.1% (PDQ‐39) and 19.3% (random data) of the total variance. Factors five to eight explained an additional 12.5% (PDQ‐39) and 16.1% (random data) of the total variance. ADL, activities of daily living; COM, communication; EMO, emotional well‐being; MOB, mobility; SOC, social support; STI, stigma.