Wikipedia:Identifying and using tertiary sources
Generally speaking, tertiary sources (for Wikipedia purposes, as discussed at Wikipedia:No original research § Primary, secondary and tertiary sources, and Wikipedia:Identifying reliable sources § Primary, secondary, and tertiary sources) include any compilation of information, without significant new analysis, commentary, or synthesis, from primary and secondary sources, especially when it does not indicate from which sources specific facts were drawn. The distinction between tertiary and secondary sources is important, because Wikipedia's no original research policy states: "Articles may make an analytic, evaluative, interpretive, or synthetic claim only if that has been published by a reliable secondary source." Thus, such claims cannot be cited to tertiary or primary sources.
Identifying
There are many types of typically tertiary sources:
- Encyclopedias, dictionaries, encyclopedic dictionaries, and compendia, whether general or topical. These are often, but not always, high-quality and reliable sources (compendia are the least likely to be acceptable).
- Coffee table books run the gamut from books written by experts and published by internationally renowned museums, to books filled with photographs of a particular place or subject, on down to books whose sole raison d'être is making people laugh.
- "Bathroom books". These are usually low-quality sources and should usually be avoided.
- School textbooks, especially below the university level in the natural sciences and below the graduate-school level in some other fields.
- Bibliographies and indexes, concordances, thesauri, databases, almanacs, travel guides, field guides, timelines, and similar works. Quality varies widely.
- Abstracts of journal articles, legislation, etc., provided by indexing services and specialized search engines (not abstracts written by the article authors themselves). Low-quality sources. May be reliable enough for basics in some cases, depending on reputability of the publisher. The abstract included atop a journal article and written by its own authors is a primary, not tertiary source.
Some of the above kinds of tertiary sources are considered forms of secondary literature in some disciplines, but remain tertiary (for most of their content) for Wikipedia's purposes including in those disciplines. Not understanding this is a common error by subject-matter experts new to Wikipedia editing.
Exceptions
The medium is not the message; source evaluation is an evaluation of content, not publication format.
- Sometimes high-quality, generally tertiary individual sources are also primary or secondary sources for some material. Two examples are etymological research that is the original work of a dictionary's staff (primary); and analytical not just regurgitative material in a topical encyclopedia written by a subject-matter expert (secondary).
- Material found in university textbooks ranges from secondary to tertiary, even in the same work, but is most often tertiary, especially at lower levels and covering more basic subjects. Textbooks intended for primary and secondary schools are almost always tertiary and, for Wikipedia purposes, reliable only for uncontentious basic material.
- Children's books of any kind are tertiary at best, often primary, and are usually unreliable sources. Especially beware citations to books about animals; the majority of them are children's books, so check to be sure. In the same class of suspect works are "adult new reader" books, and abridged large-print editions, or any other digest version.
- Some material published in general news and journalism sources (which are usually secondary) is actually tertiary, such as topical overview articles that summarize publicly-available information without adding any investigation or analysis; and sidebars of statistics or other factoids in an otherwise secondary article. (Some is also primary, such as editorials, op-eds, film reviews, advice columns, and highly subjective investigative journalism pieces.) News reporting is often mostly primary (quoting eyewitness statements, or the observations of an eyewitness reporter, rather than based on more in-depth material from experts and notable organizations). News reporting is treated more and more as if primary, regardless of what it contains, the closer it is to the date of the events, and the further in time those events recede.
- Similarly, not all documentaries aired on quasi-nonfictional TV networks are actually secondary sources; many are tertiary, and simply summarize various views of and facts about a history or science topic, without the result being novel. Some are even primary, for any exaggeratory conclusions they reach on their own. This has become increasingly true as documentary channels produce more fringe entertainment material about aliens, ghosts, ancient alleged mysteries, etc.
- Systematic reviews in academic journals are secondary sources, especially when they are themselves peer-reviewed, despite aggregating information from multiple previous publications. The less analytic kind of academic review article, the literature review, may be secondary or tertiary depending on its content.
- A review in the more general sense, of a book, film, etc., may be a primary source representing the aesthetic opinions of a reviewer, a secondary analytical piece (rarely, and most often in academic journals), or a tertiary neutral abstract of the reviewed work's content. Many are a mixture of more than one of these.
- Certain kinds of sources that are usually tertiary may in some instances be primary, e.g. rules published by a sport's governing body (primary but high-quality source) versus found in a compendium of sports and games (tertiary and low-quality, because likely to be outdated and to be missing details).
- Any tertiary source can be a primary source, when we are referring explicitly to the content of the source as such. For example, in a comparison of varying dictionary definitions, each dictionary cited is a primary source for the exact wording of the definitions it provides (e.g. if we want to quote them directly), while all of them would be tertiary sources for the meaning and interpretation of the term being defined, in a more usual editorial context.
- Some usually primary types of how-to and advice material, including user guides and manuals, are tertiary (or even secondary, depending on their content) when written by parties independent of the subject, e.g. the in-depth computer operating system guides found in bookstores (as opposed to the basic one that arrived from the manufacturer in the box with the computer).
- An abstract prepared by the author[s] of a journal paper is a primary source, like the paper itself. A summary produced by a journal's editors is secondary. A machine-produced digest is not a source at all.
- Primary source material that is simply reprinted (even with some reformatting or digesting) in an otherwise tertiary or secondary source remains primary. This includes quotations.
Determining reliability
Reliability of a tertiary source is principally determined by four factors: whether its producers (i.e. writers and/or editors) have subject-matter expertise, whether the underlying original sources of the non-novel material are clear, whether its producers are independent of the subject, and whether the work is generally regarded as reliable by others in the field in question (primarily a matter of authorial and publisher reputability). These factors counterbalance each other. For example, while typical mainstream dictionaries do not cite sources for specific entries, how authoritative they are considered can be gleaned from independent reviews of their content and editorial practices. Many tertiary works only cite sources in a general way, e.g. a bibliography. Beware tertiary works that have no indication of their own sources at all.
Another factor to consider with tertiary sources is they are often more error-prone than secondary sources, especially the more comprehensive they are. A database of millions of pieces of biographical data, each often taken from a single original primary source and added by a stressed and bored data-entry operator, is less likely to have gotten a particular individual's birth date correct than a book (secondary source) written about that person, drawing on multiple sources.
Tertiary online sources that are written in whole or in part by a general-public editing community are user-generated content, and are not reliable sources. This includes content farms, which have a paid but indiscriminate array of innumerable writers, and little editorial oversight, though many of them go to some lengths to disguise their nature.