Huh Soonim, Rice Thomas, Ettner Susan L
Korea Institute for Health and Social Affairs, San 42-14, Bulgwang-dong, Eunpyeong-gu, Seoul 122-705 Korea.
Health Serv Res. 2008 Jun;43(3):810-32. doi: 10.1111/j.1475-6773.2007.00804.x.
To identify determinants of drug coverage among elderly Medicare beneficiaries and to investigate the impact of drug coverage on drug expenditures with and without taking selection bias into account.
DATA SOURCES/STUDY SETTING: The primary data were from the 2000 Medicare Current Beneficiary Survey (MCBS) Cost and Use file, linked to other data sources at the county or state-level that provided instrumental variables. Community-dwelling elderly Medicare beneficiaries who completed the survey were included in the study (N=7,525). A probit regression to predict the probability of having drug coverage and the effects of drug coverage on drug expenditures was estimated by a two-part model, assuming no correlation across equations. In addition, the discrete factor model estimated choice of drug coverage and expenditures for prescription drugs simultaneously to control for self-selection into drug coverage, allowing for correlation of error terms across equations.
Findings indicated that unobservable characteristics leading elderly Medicare beneficiaries to purchase drug coverage also lead them to have higher drug expenditures on conditional use (i.e., adverse selection), while the same unobservable factors do not influence their decisions whether to use any drugs. After controlling for potential selection bias, the probability of any drug use among persons with drug coverage use was 4.5 percent higher than among those without, and drug coverage led to an increase in drug expenditures of $308 among those who used prescription drugs.
Given significant adverse selection into drug coverage before the implementation of the Medicare Prescription Drug Improvement and Modernization Act, it is essential that selection effects be monitored as beneficiaries choose whether or not to enroll in this voluntary program.
确定老年医疗保险受益人的药物覆盖决定因素，并研究药物覆盖对药物支出的影响，同时考虑和不考虑选择偏倚的情况。
数据来源/研究背景：主要数据来自2000年医疗保险当前受益人调查（MCBS）成本与使用文件，并与县或州层面提供工具变量的其他数据源相关联。完成调查的社区居住老年医疗保险受益人被纳入研究（N = 7525）。通过两部分模型估计预测有药物覆盖概率的概率单位回归以及药物覆盖对药物支出的影响，假设方程间无相关性。此外，离散因素模型同时估计药物覆盖的选择和处方药支出，以控制自我选择进入药物覆盖的情况，允许误差项在方程间相关。
研究结果表明，导致老年医疗保险受益人购买药物覆盖的不可观察特征也导致他们在有条件使用时药物支出更高（即逆向选择），而相同的不可观察因素不影响他们是否使用任何药物的决定。在控制潜在的选择偏倚后，有药物覆盖的人群中使用任何药物的概率比没有药物覆盖的人群高4.5%，并且药物覆盖导致使用处方药的人群药物支出增加308美元。
鉴于在《医疗保险处方药改进和现代化法案》实施之前药物覆盖存在显著的逆向选择，在受益人选择是否参加这个自愿计划时监测选择效应至关重要。
