Pennington Mark, Gomes Manuel, Donaldson Cam
King's Health Economics, King's College London, London, UK (MP).
Department of Health Services Research & Policy, London School of Hygiene & Tropical Medicine, London, UK (MG).
Med Decis Making. 2017 Aug;37(6):623-634. doi: 10.1177/0272989X17691771. Epub 2017 Feb 15.
Protest responses, whereby respondents refuse to state the value they place on the health gain, are commonly encountered in contingent valuation (CV) studies, and they tend to be excluded from analyses. Such an approach will be biased if protesters differ from non-protesters on characteristics that predict their responses. The Heckman selection model has been commonly used to adjust for protesters, but its underlying assumptions may be implausible in this context. We present a multiple imputation (MI) approach to appropriately address protest responses in CV studies, and compare it with the Heckman selection model.
This study exploits data from the multinational EuroVaQ study, which surveyed respondents' willingness-to-pay (WTP) for a Quality Adjusted Life Year (QALY). Here, our simulation study assesses the relative performance of MI and Heckman selection models across different realistic settings grounded in the EuroVaQ study, including scenarios with different proportions of missing data and non-response mechanisms. We then illustrate the methods in the EuroVaQ study for estimating mean WTP for a QALY gain.
We find that MI provides lower bias and mean squared error compared with the Heckman approach across all considered scenarios. The simulations suggest that the Heckman approach can lead to considerable underestimation or overestimation of mean WTP due to violations in the normality assumption, even after log-transforming the WTP responses. The case study illustrates that protesters are associated with a lower mean WTP for a QALY gain compared with non-protesters, but that the results differ according to method for handling protesters.
MI is an appropriate method for addressing protest responses in CV studies.
抗议性回应是指受访者拒绝表明他们对健康收益的估值，这种情况在条件价值评估（CV）研究中经常出现，并且这些回应往往会被排除在分析之外。如果抗议者在预测其回应的特征上与非抗议者不同，那么这种方法将会产生偏差。Heckman选择模型通常被用于调整抗议性回应，但在此背景下其潜在假设可能并不合理。我们提出一种多重填补（MI）方法来恰当地处理CV研究中的抗议性回应，并将其与Heckman选择模型进行比较。
本研究利用了来自多国的EuroVaQ研究的数据，该研究调查了受访者为一个质量调整生命年（QALY）的支付意愿（WTP）。在此，我们的模拟研究评估了MI和Heckman选择模型在基于EuroVaQ研究的不同现实场景中的相对表现，包括具有不同缺失数据比例和无回应机制的情景。然后我们在EuroVaQ研究中举例说明用于估计QALY收益的平均WTP的方法。
我们发现，在所有考虑的情景中，与Heckman方法相比，MI的偏差和均方误差更低。模拟结果表明，即使对WTP回应进行对数变换后，由于正态性假设的违背，Heckman方法仍可能导致平均WTP被严重低估或高估。案例研究表明，与非抗议者相比，抗议者对QALY收益的平均WTP较低，但结果因处理抗议者的方法而异。
MI是处理CV研究中抗议性回应的一种合适方法。
