01 NIS 2002-2011 Within Year Merge

<br>NIS 2002-2011 Within Year Merge - Each year of the NIS has a Core, Hospital and Severity file: File Level ID Core discharge KEY, HOSPID Hospital hospital HOSPID Severity discharge KEY, HOSPID 1. The 2 dischrage-level files will trimmed down to desired variables and then merged by KEY and saved into a temporary SAS dataset. 2. The hospital file will be trimmed and then merged into the core-severity and saved into a permanent SAS dataset with following notation: NIS_YYYY 3. Working directory cleared after every year since very large datasets.

NIS 2002-2011年度内合并数据集处理流程如下：每一年的NIS数据集均包含核心文件（Core File）、医院文件（Hospital File）与病情严重程度文件（Severity File）三类文件，其文件级标识配置为：核心出院记录的KEY与HOSPID（医院标识，Hospital ID）、医院文件的HOSPID、严重程度出院记录的KEY与HOSPID。1. 两份出院层级（discharge-level）文件将先筛选保留所需变量，随后以KEY为合并键进行合并，并保存为临时SAS数据集（SAS Dataset）。2. 医院文件将先完成变量筛选，随后与上述核心-严重程度合并数据集进行关联合并，最终以命名格式`NIS_YYYY`保存为永久SAS数据集。3. 由于单年数据集体量庞大，每完成一年的数据处理后均会清空工作目录。