It is best to use a search engine when the user:
- knows a great deal about the topic
- can narrow the topic to a specific sub-field or concept
- can identify specific words or phrases within the scope of the topic
It is best to use a directory when the user:
- knows very little about the topic
- cannot narrow the topic to specific sub-fields or concepts
- cannot identify specific keywords or phrases within the scope of the topic
Users can determine where best to begin their query by asking a few questions:
- For what am I looking?
- Are there any aspects of this topic that are particularly relevant for my search?
- What words or phrases would I expect to find in Web documents that contain the information for which I am looking?
- Are there any synonymous or equivalent words for the topic / sub-topic / concept / aspect of topic for which I am looking?
Once these questions are answered, the user can then decide which strategy and tool work best for this particular topic and his/her particular needs. Answering these questions will give the user an idea of how much she/he knows about a the topic and whether or not the terms he/she is likely to use in a search query will return results that will be relevant.
Keep in mind that it is often hard to know at the outset just which search tool will work best. Most of us start with a search and this is not necessarily the best strategy. Search functions are "dumb;" they look for the words you ask them to, not the meaning of the words. This is because search functions (be it for a search engine or a directory) are literal. If a user enters a query for a term or concept, the search engine or directory will search for the word itself, which probably appears in many documents and in wildly different contexts, some of which probably do not relate to your topic at all. In addition, unlike a library's catalog, there is no controlled vocabulary to make headings uniform. On the Web, a person can call a kitten a puppy and your search will return include this irrelevant information.
Practice is the best way for a user to recognize the best search tool and strategy for any given query. Most likely, a hybrid of searching and browsing both search engines and directories will be employed in a successful search. Another suggestion is to search different engines and directories in order to cross-reference your finds. A good rule of thumb is to do three different searches with three different search engines and/or directories.