4 Engines also cannot gather data from password sites. Other restrictions include the use of a robots.txt file or a robot meta tag. Document types may further affect an engine's ability to collect data. Consider sites using passwords for access, or proprietary software to create or index documents. None of the content from these sites resides in an engine.
Data collection comprises a portion of the factors influencing information retrieval.
Another concern entails indexing methods. It is not safe to assume that all engines index all data contained within a document. Moreover, even if an engine indexes everything, keyword research strategies are inherently fallible. Consider this example. Thomas , a Library of Congress site offering congressional documents, provides both keyword and subject term access to bill summaries.