These are documents, used in social science, which record part of a person's life—most frequently in their own words. The most obvious examples are letters, diaries, biographies and life-histories, but the term can be stretched to include many other items from photographs to inscriptions on tombstones. (The surprisingly wide sources of data are fully described in K. Plummer 's Documents of Life, 1983.) Personal documents aim to capture the subjective side of a person's life and are valuable as part of an ideographic research strategy. They are often used in the early and exploratory stages of research but can also be used as case-studies for theory generation and falsification. Personal documents were particularly popular in the work of some of the early Chicago sociologists: for example, Clifford Shaw gathered many life-histories of delinquents, and the classic study by William Isaac Thomas and Florian Znaniecki The Polish Peasant in Europe and America (1918) analysed a series of letters, as well as presenting a major life-history. See also documentary research.