township-village enterprises. One to three workers were randomly selected from each factory based on the worker name list and were invited for personal interviews. Another half of the sample is rural household heads, or farmers, who were interviewed in or around their houses. A list of communities was first randomly selected in the three municipalities, and a certain number of household interviews were assigned to each selected community. Teams of enumerators were sent to the communities, and the enumerators knocked the doors of the households selected and invited the heads of the households to participate in the interviews. If the heads of the households were not at home or refused to be interviewed, the neighbor households were then selected, until the total number of interviews in this community reached the target number.

The questionnaire was developed from a similar questionnaire previously employed in China, and was pre-tested in each of the municipalities. Special cares were paid to the WTP section of the questionnaire. Several group discussions and two pre-tests were conducted at each of the three study areas, focusing on the wording of the WTP questions as well as the price range. The survey was conducted by teams of researchers, professors and graduate students, and the survey teams were first trained by one of the authors of this paper, and then participated in the group discussions and the pre-tests of the questionnaire in order for them to fully understand the issue and to get familiar with the task before the formal surveys started. The final version of the questionnaire includes seven parts: personal characteristics, environmental perceptions and attitudes, local pollution control issues, pollution impacts on respondents and their families, household situation, health status, and finally the contingent valuation questions about cancer prevention.

The willingness-to-pay question on health risk reduction is posed in the form of cancer risk prevention. Aiming to obtain detailed information about the attitudes of each respondent toward the proposed cancer vaccine, we use a Multiple Bounded Discrete Choice (MBDC) format to check about respondent’s WTP. This format combines two aspects of development from the traditional dichotomous choice (DC) WTP question. On one hand, it allows respondents to vote on a wide range of referendum thresholds, and on the other hand, a scale of “polychotomous choice” response options from “Definitely No” to “Definitely Yes” is also provided to allow respondents expressing their levels of voting certainty for the referendum at each price level. In this way, MBDC survey technique actually reinforces both quantity and quality of CV data<sup>8</sup> .

# The WTP question in our questionnaire is as follows:

“Suppose there is a medicine that can prevent you from getting cancer for one year with one dose. There would be no side effects. But after one year, the medicine will fully stop functioning. With given quality and impact, the price of the medicine could be different. We would like to know the possibility for you to buy such a vaccine, with different prices, to make sure you would not fall ill of cancer for one year.

> speak out their answers to the interviewers. But just like with a mail survey, the final quality of the questionnaire completion can not be controlled by the enumerators.

> 8 For more discussions, see Wang and He (forthcoming).

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