“…Type of respondent can be an individual, such as Alvarez-Farizo and Hanley [12], Batley et al [13,14], Bergmann et al [15], Borchers et al [16], Botetzagias et al [17], Byrnes et al [18], Ek and Matti [10]; Goett et al [19], Groothuis et al [9], Hanemann et al [20], Hansla et al [21], Koundouri et al [22], Meyerhoff et al [23], Meyerhoff [24], Mirasgedis et al [25], Strazzera et al [26] and Zhang and Wu [27]; the head of family [28] or the homeowner [29] but scholars often refer to the "family" as a "unified" research unit, such as Bigerna and Polinori [30,31], Bollino [32], Dimitropoulos and Kontoleon [33], Grösche and Schröder [34], Kim et al [35], Ivanova [36,37], Nomura and Akay [38] and Zorić and Hrovatin [39].…”