Problems with tags

We tag items like pictures and Web posts in order to retrieve them later. Tags have several advantages over, say, a relational database or a taxonomy link. It is not necessary to fully plan the application in advance. As users add new items and new types of item that may have not been anticipated by the system designer, they can invent new tags. They do not have to worry about fitting their entry into a pre-determined category. Tagging systems can evolve as the application is used.

On the other hand, tags can be imprecise. Users may use different tags to mean the same thing. One user might tag a restaurant that averages $20 per entree as cheap and another might think that is expensive. Or, the same tag may mean different things to different users. Pasadena may refer to a city in Texas or a city in California and Capri might be an island or a Ford car model. Tags can proliferate -- a photo of a landscape might be tagged mountain or mountains or hills or sunset or landscape, etc.

One might attempt to constrain a tagging system by making agreements outside the system -- defining the vocabulary. For example, by stating that restaurants with entree prices under $10 should be tagged as cheap, those between $10 and $20 as moderate, and those over $20 as expensive. That takes care of the imprecision of the vocabulary, but it requires the same sort of planning as a relational database would. One could allow the external agreements to change -- perhaps adding a new category like very expensive or very cheap, but the users would have to agree with the changes and old entries perhaps revised. (This is the approach taken by Freebase).

Many systems show the users the tags that have been used previously when they are entering their tags. If none of the previously used tags are appropriate, the user can use a new one. This helps with the problems of using different tags to mean the same thing and with tag proliferation. Other systems constrain the number of tags that can be applied to an object. For example, the Basecamp project management system allows users to apply only one tag to a posted message.

One variation that I have not seen would be to allow users to add new tags when they feel they were needed, but to require them to also write a short description of the tag and the way they are using it. Subsequent users of that tag, cold refer to its description and even comment upon or modify it. This would help with both vocabulary imprecision and tag proliferation.

Tag clouds like this also give users guidance as to which tags are in use and are popular:

The size of the tag is an indication of how many times it has been used.

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