**1.5 Unresolved issues concerning the prevalence of NPA in patients with PD**

The results of NPA prevalence studies vary significantly. For example, Schredl et al. [33] reported an NPA prevalence rate of 37%, whereas Stein et al. [20] reported a figure almost twice as high (68%). Moreover, different authors use different frequency criteria, ranging from "at least one lifetime NPA" [34–36], to "two to four NPA per year" [33], to "a minimum of four NPA per month" [37], or to "many times per week" [33]. A summary of the data is indicated in order to gain clarity on the rate of NPA in patients with PD.
