"... As noted in a previous article in this series, there are at least three important criteria that should be used to evaluate search engines: comprehensiveness, currency, and relevance. Comprehensiveness, the measure of what fraction of the total web the search engine index actually includes, is particularly important for chemists, because they are often looking for unusual information that may not be included in smaller search engine indices. Currency measures how often the search engine revisits sites to determine whether or not there have been any changes. This is important to all web searches, since failure to revisit sites allows dead links to be included in the index. The final important criterion is relevancy. Are the most useful sites listed early in the search results?