Revisiting the Relationship between WIC Participation and Breastfeeding: An Instrumental Variable Approach

Our primary data source is the National Immunization Surveys (NIS). NIS is a national annual cross-sectional population-based survey sponsored and conducted by the National Center for Immunization and Respiratory Diseases (NCIRD) of the Centers for Disease Control and Prevention (CDC). It is an annual, telephone-based survey by interviewing with parents or guardians in all 50 states, the District of Columbia, and some U.S. territories to monitor vaccination coverage among children 19–35 months and teens 13–17 years. The NIS provides nationally, state, and locally representative estimate of vaccination coverage among children and teenagers. We used only the NIS-child data in this study, which contains information on sociodemographic and health insurance information for 19 - 35 months children as well as their mothers’ WIC participation status. NIS-child also reports the child’s breastfeeding information. NIS-child is the data used by CDC to produce the annual national Breastfeeding Report Card, which includes the national- and state-level breastfeeding rates. Aiming to be comparable and consistent with the results of the CDC Breastfeeding Report Card, we included the data from 50 states and D.C. and combined two survey years data as one cohort by the year of child-birth following the methods used in the CDC NIS data analysis guide. Data for this study were derived from the 2006-2016 NIS-child public use data, which does not provide the survey child’s birth month and year information but includes the child’s age group information (age groups: 19-23 months, 24-29 months, and 30-35 months respectively). We used the child’s age group information to infer their birth year. For example, children aged 19-23 months in the 2015 NIS-child survey year represent the child from January 2014 to May 2014. Similarly, children aged 24 months or older in the 2016 NIS-child survey data can also be born in 2014. Therefore, we combined these children from the 2015 and 2016 NIS-child survey data to create a 2014 NIS-child birth cohort. In this study, we created the birth cohorts of 2005 to 2014 by using the 2006-2016 NIS-child survey data. To ensure that these birth cohorts consistent with the cohorts in the CDC’s breastfeeding report card, we replicated the ever-breastfeeding rates, which were essentially the same as in the published report.

本研究的核心数据源为国家免疫规划调查（National Immunization Surveys, NIS）。NIS是由美国疾控中心（Centers for Disease Control and Prevention, CDC）下属的免疫与呼吸道疾病国家中心（National Center for Immunization and Respiratory Diseases, NCIRD）资助并开展的全国性年度横断面人群调查。该调查为年度性电话访谈调查，覆盖美国全部50个州、哥伦比亚特区及部分海外领地，访谈对象为儿童的父母或监护人，旨在监测19至35月龄儿童与13至17岁青少年的疫苗接种覆盖率。NIS可提供全国、各州及地方层面具有代表性的儿童与青少年疫苗接种覆盖率估算值。本研究仅使用其中的NIS儿童数据集（NIS-child），该数据集包含19至35月龄儿童的社会人口统计学信息、健康保险信息，以及其母亲参与妇女婴幼儿特殊营养补充项目（Women, Infants, and Children, WIC）的状态。NIS儿童数据集还收录了儿童的母乳喂养相关信息，亦是美国疾控中心用于编制年度《母乳喂养报告卡》的数据源，该报告卡涵盖全国及各州层面的母乳喂养率数据。为确保本研究结果与美国疾控中心《母乳喂养报告卡》的结果具有可比性与一致性，我们仅纳入50个州及哥伦比亚特区的数据，并参照美国疾控中心NIS数据分析指南中的方法，按照儿童出生年份将两个调查年度的数据合并为一个出生队列。本研究使用的数据来自2006-2016年NIS儿童公开使用数据集，该数据集未提供受访儿童的出生月份与年份信息，但包含儿童的年龄分组信息（年龄组分别为：19-23月龄、24-29月龄及30-35月龄）。我们通过儿童的年龄分组信息推断其出生年份：例如，2015年NIS儿童调查中19-23月龄的儿童，其出生时间为2014年1月至2014年5月；同理，2016年NIS儿童调查中24月龄及以上的儿童也可能出生于2014年。因此，我们将2015年与2016年NIS儿童调查中的此类儿童数据合并，构建2014年NIS儿童出生队列。本研究通过2006-2016年NIS儿童调查数据，构建了2005年至2014年的出生队列。为确保这些出生队列与美国疾控中心《母乳喂养报告卡》中的队列保持一致，我们复现了曾母乳喂养率，结果与已发表报告中的数据基本一致。