Onwujekwe Obinna, Hanson Kara, Fox-Rushby Julia
Gates Malaria Partnership, London School of Hygiene and Tropical Medicine, London WCIE 7HT, UK.
Soc Sci Med. 2005 Feb;60(3):525-36. doi: 10.1016/j.socscimed.2004.05.023.
This study's objective is to determine the factors that cause divergences over time (differences) between stated willingness to pay (WTP) and actual WTP (purchase behaviour), and consider whether any divergence signifies the presence of bias in contingent valuation studies. Stated WTP for insecticide-treated bed-nets (ITNs) was elicited from a random sample of respondents using three question formats in Nigeria. The question formats were the bidding game (BG), binary with follow-up (BWFU) and a novel structured haggling (SH) technique. The sales of the nets and a second survey were conducted 1 month after the first survey. In the second survey, factors that might explain the divergences were built into the questionnaire and these together with socio-economic variables were examined for causes of divergences in WTP. Data were analysed using non-parametric tests, testing of means and cross-tabulations. There were divergences in WTP in all three question formats: 69.4% in the BG, 78.7% in the BWFU and 48.8% in the SH. The higher the stated WTP, the more likely the divergence between stated and actual WTP. The attitude of the community leaders to the ITNs in the BG (p<0.05), the time respondents had to think about their WTP (p<0.05) and the external information they received about the ITNs in the BWFU (p<0.05) all led to divergences in WTP. We conclude that there are genuine causes of divergences between stated and actual WTP across the three question formats, and that the lesser the criterion validity score, the more the level of divergence in WTP. Studies that compare stated and actual WTP should explicitly determine the causes of divergences in order to assess the role of bias in the divergences.
本研究的目的是确定随着时间推移导致陈述性支付意愿（WTP）与实际WTP（购买行为）之间出现差异的因素，并考虑任何差异是否表明在条件估值研究中存在偏差。在尼日利亚，采用三种问题形式从随机抽取的受访者样本中获取了对经杀虫剂处理蚊帐（ITN）的陈述性WTP。问题形式包括投标博弈（BG）、后续二元法（BWFU）和一种新颖的结构化讨价还价（SH）技术。在第一次调查1个月后进行了蚊帐销售及第二次调查。在第二次调查中，可能解释差异的因素被纳入问卷，并将这些因素与社会经济变量一起考察WTP差异的原因。数据采用非参数检验、均值检验和交叉表分析。在所有三种问题形式中WTP均存在差异：BG中为69.4%，BWFU中为78.7%，SH中为48.8%。陈述性WTP越高，陈述性与实际WTP之间出现差异的可能性就越大。BG中社区领袖对ITN的态度（p<0.05）、受访者思考其WTP的时间（p<0.05）以及BWFU中他们收到的关于ITN的外部信息（p<0.05）均导致了WTP的差异。我们得出结论，在这三种问题形式中，陈述性与实际WTP之间存在差异是有真正原因的，并且标准效度得分越低，WTP的差异程度就越大。比较陈述性与实际WTP的研究应明确确定差异的原因，以便评估偏差在差异中所起的作用。
