Compares the claims of the patent in question to claims of unexpired patents and details of other types of prior art. A validity search is sometimes undertaken to determine if a patent is actually valid and in force. This type of search is essentially an example of the use of patents as a competitive or strategic weapon. The idea is to locate "prior art patents or other facts (italics added) which will anticipate the claimed subject matter of the problem patent or will render it obvious to one of ordinary skill in the art, either of which would invalidate the patent". In this type of search, other materials can be particularly helpful, such as "old sales catalogs, trade journals, (and) prior literature".