Wikipedia:Identifying and using tertiary sources

Generally speaking, tertiary sources (for Wikipedia purposes, as discussed at, and ) 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 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 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 (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 editions, or any other digest version.
 * Some material published in general news and journalism sources (which are usually primary) 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  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.

Usually acceptable uses
Simple facts: A tertiary source is most often used for reference citations for basic and fairly trivial facts which are not likely to be disputed and which can be verified in other sources. Examples include various vernacular names for a species, the pronunciation of a foreign word, or a baseball player's statistics in a particular year. The WP:Good article nominations and WP:Featured article candidates processes tend to check that statements in an article are sourced, and tertiary sources frequently are used for many non-controversial details.

Simple comparisons: Another common use is comparative, especially involving simple facts and basic concepts. An example is citing multiple dictionaries to show how interpretation of a term may vary. (Comparative use of tertiary sources for more complex or contentious material is ill-advised, as detailed below.)

Establishing balance: Tertiary sources are often included as supporting (not principal) evidence in considering the relevant prominence and due weight of conflicting views, to ensure article balance.

Better than nothing: Tertiary sources are also commonly used when a secondary source has not yet been found. For example, a field guide about cacti has probably been reasonably well fact-checked, and can be cited as a source for the range of a particular species, if no source focusing on that species (and perhaps with more recent data) has turned up yet.

Older but still relevant details: Older tertiary sources can be used to source former, obsolete views or facts that need to be reported on in a Wikipedia article, for completeness, especially when it's difficult to find modern sources that even mention a long-replaced idea, name, person fulfilling a role, or whatever. As detailed below, there is a major difference between using a tertiary source to report obsolete facts as such, and trying to use them to preserve obsolete facts as still verifiable (e.g., you can use 19th-century encyclopedias to illustrate how seriously phrenology was once taken, but such sources cannot be used to try to contradict modern scientific works).

Problematic uses
Analysis and evaluation: A tertiary source cannot be used, as a matter of policy, as a source for "an analytic or evaluative claim". This is left deliberately broad, so it is not subject to technicality gaming.

Controversial material: Any controversial, alleged fact is essentially unsourced if the only citation it has is to a tertiary source of questionable reliability (on the particular point or generally). As with secondary sources, this can happen for any number of reasons, including source obsolescence, lack of subject-matter expertise, conflict of interest, simple error, or presentation of a fringe idea as comparable to the generally accepted view, among other problems that can arise with a particular source. A tertiary source that is a compendium of factoids by an author with no known expertise, and which indicates nothing about the sources of its own information, is not a reliable source. Anyone could compile a large book of alleged facts, anecdotes, and folklore about any given topic, and probably find a willing publisher, without any fact-checking ever taking place. Note however that not all facts about a controversial subject are themselves controversial; there is no principle that a reliable tertiary source good enough for one article is not good enough for another because of the topic's notoriety, the amount of emotion editors bring to editing it, or the frequency with which our article on it is vandalized.

Complex or controversial comparisons: Comparative use of tertiary sources can be fraught with problems relating to undue weight, non-neutral point of view, novel synthesis, and lack of basic accuracy if the things being compared are subject to real-world contention, or are complex in nature. For example, a comparison between Christian, Judaic, and Muslim concepts of God is unlikely to produce encyclopedic results if based in whole or part on tertiary sources, which are likely to present a poorly nuanced view of complex theological questions and details of interpretation. Complex comparative work must actually be done in secondary sources cited by Wikipedia for those comparisons. The WP:AEIS policy does not permit Wikipedians themselves to engage in substantive "analysis, evaluation, interpretation, or synthesis" of facts or sources.

Over-inclusive works: Indiscriminate sources must be considered skeptically when determining both notability and due weight. Unfortunately, a large proportion of tertiary sources are indiscriminate. A guidebook that attempts to describe every restaurant in a city cannot reasonably help establish that a particular restaurant is notable. An index of every paper published about a topic in a given year tells us nothing about the critical reception of any given paper. The more inclusive, comprehensive, even "complete" that a work aims to be, the less useful it is for determining the notability of any subject it mentions. On the up side, the more comprehensive a work is, the more likely Wikipedia editors are to find reliable details in it about any subject within its purview. Thus, in a selection of tertiary sources for a topic, the source that is most reliable for WP:Verifiability purposes has a tendency to be the least valuable for notability and due weight analysis. The inverse is often not true; an exclusively selective, non-comprehensive source may well be unreliable, too, simply because it was poorly researched and reflects a superficial, popular-opinion approach to its topic, as is often the case with coffee table books.

Better sources available: While a good tertiary source can usually be used without incident to source non-controversial facts, such citations can and should be superseded by ones to reliable secondary sources. WP:Identifying reliable sources tell us: "Each source must be carefully weighed to judge whether it is reliable for the statement being made and is the best such source for that context." It is extremely rare for a tertiary source to be the best such source, for anything, in any context; they're simply often the most readily available and easily digestible (being somewhat predigested). Sometimes a tertiary source can even be replaced with a primary one; for example, a dog breed's actual breed standard (the primary source) is more reliable for the breed's defined characteristics than a tertiary dog breed encyclopedia, though the latter might be very useful for differences and commonalities between varying standards published by different organizations, and may be a good source of additional details, like demographics and breed history. "Stacking" tertiary source citations after a sufficient secondary one is not advised; it does not add more verifiability to the claim in the article, but simply adds clutter.

Outdated material: An obsolete source cannot be used to "trump" newer reliable sources that present updated information, most especially when the older source states or implies a negative that cannot be proven but can be disproven easily by new data. A pertinent example (detailed here) is a prominent dictionary asserting that a specific phrase was first used in publication in a certain year, while later research found older examples, disproving this assertion (with its implicit negative, that there were no earlier cases). Because most tertiary works take a long time to assemble, or (in more dynamic media) are in a constant state of being incrementally updated, it is fairly likely that some particular pieces of information in such a work have already been surpassed by the newer work of others. Some information in tertiary sources may already be obsolete before they even see publication. Sometimes the very conceptual framework behind such a work becomes obsolete, given the passage of enough time, with enough advancement and reorganization in the field to which it pertains. E.g., a decades-old tertiary list of species within a genus, based on outmoded ideas of classification, cannot be used to contradict or seek undue weight against a widely accepted re-classification arrived at through more modern research. On the other hand, a recent high-quality tertiary source with clear and reliable sources may be of more value than an obsolete secondary one, especially in the sciences, where current understanding can be a fast-moving target.