Replication Data for: An Empirical Justification for the Use of Draft Lottery Numbers as a Random Treatment in Political Science Research

Abstract: Over the last several years, there has been growing use of the draft lottery instrument to study political attitudes and behaviors (see, for example, Bergan 2009; Davenport 2014; Erikson and Stoker 2011, Henderson 2012). Draft lotteries, held in the United States from 1969 to 1972, provide a potentially powerful design; in theory, they should provide true randomization for the “treatment” of military service or behavioral reactions to the threat of such service. However, the first draft lottery conducted in 1969 was not conducted in a random manner, giving those citizens born the fourth quarter of the year disproportionately higher chances of being drafted. In this note, we describe the randomization failure and discuss how this failure could in theory compromise the use of draft lottery numbers as an instrumental variable. We then use ANES data to provide support for the conclusion that individuals most affected by the randomization failure (those born in the fourth quarter of the year) largely do not look statistically distinct from those born at other times of the year. With some caveats, researchers should be able to treat the 1969 draft numbers as if they were assigned at random. We also discuss broader lessons to draw from this example, both for scholars interested in using the draft lottery as an instrumental variable, and for researchers leveraging other instruments with randomization failures. Specifically, we suggest that scholars should pay particular attention to the sources of randomization failure, sample attrition, treatment and dependent variable selection, and possible failure of the exclusion restriction, and we outline ways in which these problems may apply to the draft lottery instrument and other natural experiments.

摘要：过去数年间，征兵抽签（draft lottery）工具在政治态度与行为研究中的应用愈发广泛（参阅Bergan 2009；Davenport 2014；Erikson与Stoker 2011、Henderson 2012）。美国于1969至1972年间实施的征兵抽签，为相关研究提供了极具潜力的研究设计；理论上，其可对「服兵役」这一干预措施，或是针对服兵役威胁所产生的行为反应，实现真正的随机分配。然而1969年首次开展的征兵抽签并未严格遵循随机流程，导致第四季度出生的公民被征召入伍的概率偏高。在本研究短讯中，我们首先阐述了此次随机分配失效的具体情形，并从理论层面探讨了该失效将如何削弱征兵抽签编号作为工具变量（instrumental variable）的适用性。随后我们借助美国全国选举研究（ANES, American National Election Studies）数据，验证了受此次随机分配失效影响最显著的群体（第四季度出生者），在统计特征上与其他季度出生的个体并无显著差异。在满足部分前提条件的前提下，研究人员可将1969年的征兵抽签编号视作随机分配所得。我们还探讨了本案例可提炼的一般性经验教训，既面向有意将征兵抽签作为工具变量开展研究的学者，也面向借助其他存在随机分配失效的工具变量进行研究的科研人员。具体而言，我们建议学者需重点关注随机分配失效的来源、样本损耗、干预措施与因变量的选择，以及排他性约束（exclusion restriction）可能失效的问题，并梳理了上述问题如何适用于征兵抽签工具与其他自然实验场景。