Each pass through the evidence is provisional or incomplete. The concepts are abstract, but they are rooted in the concrete evidence and reflect the context. As the analysis moves toward generalizations that are subject to conditions and contingencies, the researcher refines generalizations and linkages to reflect the evidence better. For example, a historical-comparative researcher believes that historical reality is not even or linear; rather, it has discontinuous stages or steps. He or she may divide 100 years of history into periods by breaking continuous time into discrete units or periods and define the periods theoretically. Theory helps him or her identify what is significant and what is common within periods or between different periods. As Carr (1961:76) remarked, “The division of history into periods is not a fact, but a necessary hypothesis.” The breaks between periods are artificial; they are not natural in history, but they are not arbitrary.
The researcher cannot determine the number and size of periods and the breaks between them until after the evidence has been examined. He or she may begin with a general idea of how many periods to create and what distinguishes them, but will adjust the number and size of the periods and the location of the breaks after reviewing the evidence. He or she then reexamines the evidence with added data, readjusts the periodization, and so forth. After several cycles, he or she approximates a set of periods in 100 years on the basis of successively theorizing and looking at evidence.