## 2 Materials and Methods  
### Survey Implementation and Data Collection  
In order to collect CV data, the method of deriving the WTP from respondents; the payment vehicle, unit, and period; the method of survey; and the sample size had to be determined [17]. First, out of four methods of open-ended questions, bidding game questions, payment card questions, and dichotomous choice (DC) questions, which have been used in the literature as methods of eliciting WTP, this study adopted the DC question method. This was because the DC question method has been most frequently employed in the literature and possesses various merits, such as incentive compatibleness and mitigation of the respondents' cognitive burden [18]. In addition, Korea Development Institute [19] and [PERSON] et al. [20] present methodological guidance to be followed in applied CV research. For example, the survey correctly explained to respondents that there exist substitutes for the good, that the respondents' income is limited, and that consumption of other goods should be reduced to pay the WTP they have reported. In addition, this survey evaluated one of the many projects that the government should undertake. As will be explained below, this study tried to follow most of these guidelines. Six important points had to be determined in order to conduct an actual field survey with a well-made CV questionnaire. First, the method of survey should be determined. This study adopted a person-to-person interview, which can facilitate the delivery of information rather than utilizing a relatively low-cost telephone, mail, or Internet method. In addition, the survey was conducted by experienced interviewers belonging to a professional opinion research institute. Second, the size of the sample had to be determined. In this study, the population was all households in South Korea, and the population size is 19,971,359. The appropriate sample size had to be determined from this, with a 95% confidence level usually considered. In addition, a sample error of 5% is widely applied in South Korea, but a sample error of 3.1% was adopted in this study for more rigidity. The appropriate size of the sample was thus derived as approximately 1000, and 1000 observations were collected for the final analysis. Although a larger sample is better, it is important to size the sample at an appropriate level because the cost of the survey increases accordingly. In this regard, the size of the sample was set at 1000 following the suggestion of Korea Development Institute [19] and [PERSON] et al. [20]. In particular, since the costliest method of a person-to-person individual interview was conducted in this study, the sample size of 1000 was considered large enough and appropriate. Third, the unit of the survey had to be determined. In this study, households were chosen out of individuals and households. This was because Korea Development Institute [19] proposed the use of households as a unit of the CV survey. In addition, conducting surveys of individuals may cause an issue as to what to do with the population when expanding the sample value to the population value. In other words, whether to include people under 20 or over 65 years of age who may lack economic ability can have a significant impact on the analysis results. On the other hand, conducting a survey of households is free of this issue. To improve the reliability of the data, the participants from households were limited to the household owner and his/her spouse, who have the actual burden of tax payment. Fourth, the payment period must be determined. Naturally, the longer the payment period, the larger the total WTP, and the shorter the payment period, the smaller the total WTP. Therefore, it is important to reasonably determine the payment period. In this survey, payment was due for the next 10 years. This was because this period has been used in most applied CV works conducted in South Korea. Fifth, the payment vehicle had to be fixed. The payment vehicle was decided as the yearly household income tax.