This research grouped the innovations listed in Appendix A into four different historical eras: Pre-Twentieth Century, Early Twentieth Century (1900–1929), Mid-Twentieth Century (1930–1959), and Late Twentieth and Early Twenty-First Centuries (1960–2006). To classify policies by historical era, this research employed the procedure suggested by Walker (1969), placing a policy in the period when the tenth state adopted the innovation. For example, if the first nine states adopted an innovation in the 1920s, but the tenth state implemented a policy in 1931, the policy was included in the groups of policies for the mid-twentieth century. To confirm the reliability of this sorting procedure, this research performed a secondary sorting of policies by first taking the mean year of adoption for all states for a given innovation, and then subtracting one standard deviation from this mean year to find the period of early adoption. This procedure produced a grouping of policies by era almost identical to the protocol suggested by Walker. This process identified 17 policies that diffused prior to 1900, 28 policies in the 1900–1929 period, 35 policies in the 1930–1959 period, and 52 policies in the 1960–2006 period.