The present study is devoted to the verification of the basic metrical characteristics of the ISAPD. One hundred and sixty-seven Parkinson's disease (PD) patients were included. Group A (n = 40) was simultaneously assessed by five raters who applied the ISAPD and other PD rating scales (PDRS). A set of timed tests, the MiniMental State Examination (MMSE) and the Hamilton Scale for Depression (HSD) were administered by an independent examiner. Group B (n = 127) was individually assessed through the UPDRS and the other PDRSs by separate neurologists in four different hospitals. The ISAPD was administered in 7.0 ± 3.7 min. The internal consistency of this scale was high (Cronbach's alpha = 0.97). The inter-rater reliability of their items was very satisfactory (for all items, kappa > 0.70). There was a high correlation with the Hoehn and Yahr classification (rs = 0.71; p < 0.001) and some timed tests. The convergent validity with the other PDRS (UPDRS and Schwab and England Scale) was also very high (rs = 0.83-0.92; p < 0.001). The ISAPD also correlated with the MMSE and the HSD. Factor analysis identified three factors (activities of daily living; gait and mobility; speech and eating) that explained 76% of the variance. The ISAPD is an easy to apply, reliable, and valid scale that fulfills the aim for which it was designed.