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Mooers' law is an empirical observation of behavior made by American computer scientist Calvin Mooers in 1959. The observation is made in relation to information retrieval and the interpretation of the observation is used commonly throughout the information profession both within and outside its original context.
An information retrieval system will tend not to be used whenever it is more painful and troublesome for a customer to have information than for him not to have it.
Mooers argued that information is at risk of languishing unused due not only on the effort required to assimilate it but also to any fallout that could arise from the discovery of information that conflicts with the users personal, academic or corporate interests. In interacting with new information, a user runs the risk of proving their work incorrect or even irrelevant. Instead, Mooers argued, users prefer to remain in a state of safety in which new arguments are ignored in an attempt to save potential embarrassment or reprisal from supervisors.
The more commonly used interpretation of Mooers' law is considered to be a derivation of the principle of least effort first stated by George Kingsley Zipf. This interpretation focuses on the amount of effort that will be expended to use and understand a particular information retrieval system before the information seeker 'gives up', and the Law is often paraphrased to increase the focus on the retrieval system:
The more difficult and time consuming it is for a customer to use an information system, the less likely it is that he will use that information system.— J. Michael Pemberton
Mooers' Law tells us that information will be used in direct proportion to how easy it is to obtain.— Roger K. Summit 
In this interpretation, "painful and troublesome" comes from using the retrieval system.
- Peter Morville (2005). Ambient findability. O'Reilly Series. Marketing/Technology & Society (illustrated ed.). O'Reilly Media. p. 44. ISBN 978-0-596-00765-2.
- Mooers, Calvin. "Mooers Law, or Why some Retrieval Systems are Used and Others Are not". Business Library. Retrieved 25 October 2011.
- Austin, Brice (June 2001). "Mooers' Law: In and out of Context". Journal of the American Society for Information Science and Technology 25 (8): pp 607–609. doi:10.1002/asi.1114. Retrieved 2007-05-23.
- Calvin N. Mooers Papers, 1930-1992 at the Charles Babbage Institute, University of Minnesota.
- Oral history interview with Calvin N. Mooers and Charlotte D. Mooers at the Charles Babbage Institute. Interview discusses information retrieval and programming language research from World War II through the early 1990s.
- Another empirical observation with a similar-sounding name is Moors' law, named for Kent Moors of Duquesne University, which states crude oil prices double every five years.