I recently upgraded WordPress - the software that runs this site - to version 2.3. New in this latest version is support for tags. Each post can be associated with any number of keywords or key phrases called tags, and navigation elements can be added to display the set of tags. Probably the most commonly used tag navigation element is a tag cloud which displays the list of tags in paragraph format with more frequently-used tags displayed in a proportionally larger font. See the image below.
WordPress previously only supported categories, which I used on this site. WordPress supports associating a post with any number of categories, just like tags. So how do they differ? The first major difference is that WordPress expects categories to be predefined, whereas it allows new tags to be added without restriction. The second major difference is that the navigation element for categories is most frequently a list. Since a category list takes up much more vertical space than a tag cloud for the same number of elements, there is the expectation that the total number of categories will remain small. See the image below.
Comparing the two images, we see that the tag cloud actually takes less vertical space despite showing four times as many items. The tag cloud does use more horizontal space, but this is actually an advantage: the format of the tag cloud allows it to use almost all of the available horizontal space, unlike the category list. The tag cloud, therefore, uses the screen real estate more effectively.
Which format is more usable? The main goals of both the category list and tag cloud are to (1) provide an overview of what the site is about, and (2) help users navigate to articles of interest. I believe that the tag cloud does a better job of achieving both goals. Displaying so many more tags than the category list gives a much more complete overview of the content available on the site. There is a risk that too many tags will cause the more important content to be lost in the crowd, but this is offset by the use of larger fonts for more frequently-used tags. I suspect that navigation is generally easier for users looking for a particular topic because it is much more likely there will be a tag that corresponds to their topic. If there isn't, they do have more tags to navigate through compared to a category list, which is a disadvantage. But on average there are less articles per tag then there are per category, so they can try several tags fairly quickly. For myself, I often look up old articles while writing new ones, and I find it much easier to navigate to a specific old article via a tag than via a category. Why? I often have difficulties remembering exact what 'vague' category I filed an article under, while there is usually at least one tag I know is linked to the article.
This brings me to what I believe is the most significant advantage of tags over categories: tags correspond more closer to how our minds store and retrieve information. Categories imply a hierarchical, structured way of organizing information, with each post usually being filed in a single category. Tags imply a more arbitrary network of relationships between articles which supports multiple ways of categorizing the content. I have often struggled in the past trying to assign a post to a single category, especially when part of the post is about another category. Assigning multiple tags is much more natural in this situation.
I haven't seen any consensus on the web regarding whether tags or categories are better. Among popular social bookmarking sites Digg uses categories and Technorati uses tags. An article on Problogger about tags and categories and the resulting discussion revealed a diverse set of opinions; the article itself stated that tags compliment categories and both can co-exist on a site.
I myself am leaning towards eliminating the use of categories on my site and just using tags with the tag cloud. I would be interested in hearing your opinions on the matter. Do you prefer the tag cloud or the category list? Should I keep both? Should the tag cloud display less tags, or have a smaller size for the largest font? Please leave a comment and let me know.
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