"Bayes' Theorem" or "Bayes' Rule", or something called Bayesian reasoning
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Bayes' Theorem
  	 

Let  and  be sets. Conditional probability requires that 
 	(1) 
where  denotes intersection ("and"), and also that 
 	(2) 
Therefore, 
 	(3) 
Now, let 
 	(4) 
so  is an event in  and  for  , then 
 	(5) 
 	(6) 
But this can be written 
 	(7) 
so 
 


This paper proposes a new measure called scaled inverse document frequency (SIDF) which evaluates the conditional specificity of query terms over a subset S of D and without making any assumption about term independence. S can be estimated from search results, OR searches, or computed from inverted index data. We have evaluated SIDF values from commercial search engines by submitting queries relevant to the financial investment domain. Results compare favorably across search engines and queries. Our approach has practical applications for `real-world’ scenarios like in Web Mining, Homeland Security, and keyword-driven marketing research scenarios.