Wikipedia:Categorization and subcategories

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✔ This page documents an English Wikipedia editing guideline. It is a generally accepted standard that editors should follow, though it should be treated with common sense and the occasional exception. When editing this page, please ensure that your revision reflects consensus. When in doubt, discuss first on the talk page.
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In straightforward cases an article should not be in both a category and its subcategory. For example, Golden Gate Bridge is in Category:Suspension bridges, so it should not also be in Category:Bridges. However, there are some articles which should be in both a subcategory and a parent category.

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[edit] Test questions

Ask yourself the following questions to help decide if duplicate categorization is appropriate:

  1. Are there any explanations posted in the categories that explain what goes in each category? Many categories will say right on the top of the page what belongs in the category. Sometimes there is a discussion on the talk page that can help you make a decision. If there is nothing mentioned go on to the next question. Hopefully all categorization schemes that have duplications will have instructions directing editors about how things should be categorized.
  2. Are other articles that are in subcategories also in the higher category? If there are no others and the category and its subcategories are well populated that means that all the entries should just be in sub-categories. An example of this is Category:Entertainers. If there are entries, go on to the next question.
  3. Does it look like the articles in the parent category might be there by mistake? Sometimes there are actors in Category:Entertainers, but the few that are there have been put there by mistake and most likely fairly recently. All the other actors have been put in subcategories and all that is left is a small random selection of entries. However if there is a broad selection of entries, then you have an article which might qualify to be in both the category and subcategory.

[edit] Reasons for duplication

There are some good reasons why duplication is not a problem, but a benefit:

  1. The basic principle is the duplication makes it easier, not harder, for users to find articles.
  2. Different users may think of different categories when they look up their targets, multiple listing or locations enable users to spot their target easily
  3. Wikipedia's categorization scheme allows for multiple taxonomies. This is a good thing and a powerful feature. The categories act as indexes, facilitating users to browse through subjects.
  4. There might be articles that belong in a subcategory, while not belonging in a parent category; without duplication, users might assume that all articles in the subcategory belong in the parent category, when this may not actually be the case. (See the "user benefit" rule in the next section.)

[edit] When to duplicate

When deciding for or against duplication, one should bear the above reasons and benefits in mind, in order to weighing up your decision. It is not essential that there be consistency across all of Wikipedia's categories since each case or situation may differ. It is important that the duplication makes sense for the subject matter. Here are some situations where it is true:

[edit] Incomplete sets of subcategories

Often an article could belong in more than one subcategory of a major category, but not all of these subcategories will necessarily exist. For example, actor Marlon Brando is in Category:Best Actor Academy Award winners and its parent Category:Film actors.

While it is possible to add Category:Actors who never won an Oscar to complete the categorization scheme with all actors being in one or the other of the subcategories and none in the parent category, this wouldn't make the categories any more useful, and would make it much harder to categorize actors and search for them in categories.

Another example of this is Category:Bridges in New York City and Category:Toll bridges in New York City. ALL the toll bridges are listed in both categories. These situations come about when one hierarchy of categories (Category:Toll bridges in the United States) is a subset of another hierarchy of categories (Category:Bridges in the United States). It might be useful to visualize a Venn diagram in such circumstances.

In a sense the subcategories are related categories and not actually part of the same hierarchy. It also makes it easier to see a complete list of the bridges in each location.

[edit] Secondary categorization rule

Sometimes an article will belong in both a subcategory and its parent category because most readers will naturally think associate the subject with the parent category before the subcategory, or will consider the parent category to be a more important way of understanding the subject. A good example of this phenomenon is with categorization by nationality and ethnicity -- for instance, most articles in Category:African American baseball players should also exist in Category:American baseball players. See Wikipedia:Categorization/Gender, race and sexuality for more information.

[edit] Topic article rule

When an article is the topic article for a category, articles should be placed in the category with the same name. However, the article and the category do not have to be categorized the same way. The article belongs in categories populated with similar articles. The category should be put into categories populated with similar subcategories. For example, see George W. Bush and Category:George W. Bush.

When an article and the subcategory with the same name end up in the same category, the double listing sends the message to the user that there is an article about the topic, and there are also more articles to be found in the subcategory of the same name. It makes it easier to find main topic articles (by eliminating having to go to the subcategory). It also creates a complete listing of articles at the higher level category. It points readers of the topic article to the category and vice versa.

[edit] User benefit rule

Does the removal of duplication affect the reader, making it hard to browse through subjects or spot their target easily? If the answer is yes, you should not remove the duplication.

[edit] Category page rule

Are there any detailed explanations posted in the category pages that guide people step by step what goes in each category? Many categories will say right on the top of the page what belongs in the category, and how articles should be categorised. Sometimes reading discussions on the talk page helps in making the right decision.

[edit] Common sense rule

If you are still not sure whether duplicate categorization is appropriate after going through all the above, the best way is to use one's common sense or own judgment.

For example, when we look at Category:Musicals and its subcategory Category:Musical films. Some musicals have separate articles for the stage and screen productions and sometimes the same article describes both. Since the articles are not written in a consistent way, the duplication helps users find similar articles when they are browsing through either category.

Note: The Common Sense Rule should only be used as a last resort; when all other rules fail to determine whether duplicate categorization is appropriate then the Common Sense Rule may be invoked.