One such model is the UNISIST model of information dissemination. Within such a model these concepts are defined in relation to each other, and the acceptance of this way of defining the concepts are connected to the acceptance of the model. Some other modern languages use more than one word for the English word "source". German usually uses "Sekundärliteratur" ("secondary literature") for secondary sources for historical facts, leaving "Sekundärquelle" ("secondary source") to historiography. A "Sekundärquelle" is a source which can tell about a lost "Primärquelle" ("primary source"), such as a letter quoting from minutes which are no longer known to exist, and so cannot be consulted by the historian. In general, secondary sources are self-described as review articles or meta-analysis. Primary source materials are typically defined as "original research papers written by the scientists who actually conducted the study." An example of primary source material is the Purpose, Methods, Results, Conclusions sections of a research paper (in IMRAD style) in a scientific journal by the authors who conducted the study. In some fields, a secondary source may include a summary of the literature in the Introduction of a scientific paper, a description of what is known about a disease or treatment in a chapter in a reference book, or a synthesis written to review available literature. A survey of previous work in the field in a primary peer-reviewed source is secondary source information. This allows secondary sourcing of recent findings in areas where full review articles have not yet been published. A book review that contains the judgment of the reviewer about the book is a primary source for the reviewer's opinion, and a secondary source for the contents of the book. A summary of the book within a review is a secondary source. In library and information sciences, secondary sources are generally regarded as those sources that summarize or add commentary to primary sources in the context of the particular information or idea under study. An important use of secondary sources in the field of mathematics has been to make difficult mathematical ideas and proofs from primary sources more accessible to the public; in other sciences tertiary sources are expected to fulfill the introductory role. Secondary sources in history and humanities are usually books or scholarly journals, from the perspective of a later interpreter, especially by a later scholar. In the humanities, a peer reviewed article is always a secondary source. The delineation of sources as primary and secondary first arose in the field of historiography, as historians attempted to identify and classify the sources of historical writing.