“…Dichotomous choice questions can be broken down into single- or double-bounded questions, where a double-bounded question means that, after being given an initial ‘yes or no’ WTP price, as in a single-bounded question, the respondent is then given a second WTP option dependent on his first answer [32]. The most popular question format of the 22 papers is an open-ended question (48%) [11, 15, 16, 19, 27, 29, 30, 33, 47], followed by dichotomous choice [1, 15, 17, 26, 40, 47, 54, 55] (35%), and payment card [33–37]. Two of the papers use both open-ended questions and dichotomous choice [15, 47].…”