Precision implies retrieving a small set of results which best match the information query or need. Because a subject- oriented approach limits possible responses to a relevant category, precision tends to be high there. However, because the subject- oriented approach is selective, it can not always claim to be comprehensive. Nor is it always likely to include responses of peripheral interest which do not quite fit a given category. Recall implies retrieving a larger set of responses to ensure that closely related results are included, thus ensuring comprehensiveness.
Relevance is determined by subject expertise.
In the subject-oriented approach, subject knowledge is applied by those who construct the resource to ensure relevant sources are brought together in one place. When using larger, less specific search engine databases, the subject expertise must be applied by the end user. Thus, it is often more appropriate to guide beginning level users to subject-oriented resources, while experts in a given subject area are more likely to have success with large databases. They offer the convenience of providing access to a large pool of possible of results. They promise the user the ease of having to learn only one search engine interface.