A top-down strategy assumes you are designing a topic from the top-level topics down through the individual evidence topics of each subtopic. To design from the top down, you must adopt a taxonomy, or scientific classification approach, to creating a topic, as follows:
Top-level topics: use general headings to identify the subject area
Subtopics: use more specific headings to identify the primary groupings within the subject area, as well as topics which are increasingly more specific.
Evidence topics: use important terms, acronyms, or jargon, to define the subject.
A top-down design works best when you have clearly-defined requirements. This approach is also ideal if your set of searchable documents is constantly growing or changing. With this strategy, for example, you are likely to define subjects which may not yet be evident in your information sources. Keep in mind that you can always add new topics, if you find that a number of new documents contain information which are not identified in your topic design.
If your information sources (that is your set of indexed documents) changes constantly, specific subjects within documents may be missed, especially at the lowest levels. So, you should periodically analyze the information being selected by your topics to ensure that topics critical to your application are current, and the appropriate information is being found.