Tag hierarchy

There's been some discussion about the effectiveness of the topic hierarchy on Know Between.

The topics are designed to associate questions with relevant subject areas, so that they can target people who might be interested in or able to answer them, and award credibility accordingly.

Open tags
Initially, topics were just a sea of open tags. Users were able to choose whatever tags they wanted without further questioning. The drawback was that there was no relationship between tags and a lot of duplication. For instance, 'UK' and 'United Kingdom' were different; 'Birds' and 'Oystercatchers' were not connected in any way. The whole concept of building credibility and ranking for topics was in jeopardy.

Fixed hierarchy
A fixed hierarchy is commonly adopted to overcome the problems of open tags (see Yahoo! Answers and LinkedIn). The drawback here is that you're forced to limit yourself to a predetermined set of topics. There's no room for dynamic adaption of the hierarchy.

Infer relationships between tags
Another way to realise the benefits of a hierarchy or relationships without enforcing fixed categories is to provide some sort of smart, automatic matching. Flickr uses 'clusters' to establish related tags, and they provide an API to this feature which Know Between uses.

Auto-tagging
Yahoo! offer a service to extract significant words or phrases from a larger piece of content. The Term Extraction service may be useful at identifying probable tags from a question, although it will never return words that aren't in the piece of text.

Dynamic hierarchy
For Know Between we have chosen to put in place a system whereby users can choose their own tags or browse an existing hierarchy. If not browsed through the existing hierarchy, users are prompted to confirm the parent category for each tag that matches an existing one. For new tags, users are prompted to choose where in the existing hierarchy the new tag should be placed.

An initial top-level category list starts things off. These cannot be selected as topics, only as category parents. This has been a bit confusing for some, but the intention is to ensure more specific tags are chosen. Over time there will be a large set of topics that have dynamically been added. There will be fewer and fewer new topics, so less of a barrier for people asking questions.

How do you think the tagging of topics to questions can be improved?