In addition to the performance measure, we classified the studies based on the level of analysis, the source of the performance measures, the type of HR practices examined, and the types of control variables used. Level of analysis
was coded as the firm (corporation), business unit (SBU), or plant (site). The source of the performance measure was classified as being either via survey, company records, or publicly available information. The HRM variables were grouped into six categories: work organization, high performance work systems (HPWS), strategic HRM (SHRM), participation and motivation, training and selection, and compensation. The studies were also coded as to control variables used in order to provide guidance regarding what kinds of control variables seem most popular and/or appropriate. The purpose of the data collection was not to perform a meta-analysis to determine a population effect size, but rather to simply assess areas of opportunity for further research and to see what the empirical research to date reveals about an implicit meaning of performance for SHRM.