Historiography has recently been defined as "the study of how history has been and is written—the history of historical writing," which means that "when you study 'historiography,' you do not study the events of the past directly, but the changing interpretations of those events in the works of individuals..." Historiography includes studies of primary sources such as letters, documents, and other materials used by historians' subjects to record their experiences over time. It also includes works by important historians that analyze these sources or contemporary accounts of major events. The historiography of women's historiography concerns itself with the ways in which women have contributed to historiography. Women have always played an important role in society as writers, researchers, and educators. However, it was not until the 20th century that they began to be included in official histories as contributors rather than as missing figures. Although there are many notable women historians today, including Elizabeth Eisenstein, Linda K. Kerber, and Susan Myer, women's roles in history were not considered important enough to include in most histories until recently. Women have often been excluded from history books because they did not hold public office, their stories cannot be told using only secondary sources (such as archives or memoirs), or they did not leave behind writings that describe their lives. Historians have also tended to focus on men because they believed that only they could write history worth reading or quoting.