A system and methods and user interface are disclosed for searching documents based on conceptual association, and for ranking documents based on content characteristics. A computer processing system receives a query containing a word or phrase that is a name of an object or concept, and can also receive a value for a customized ranking parameter. A computer storage device stores a dataset related to the object or concept name, wherein the dataset includes a plurality of property names and can also include association-strength values. A computer processing system can count a first frequency of a first property name and count a second frequency of a second property name in a document containing text in a natural language, calculate a relevance score as a function of the first frequency and the second frequency, and rank the plurality of documents using their respective relevance scores, and return one or more documents to the user based on the ranking of the plurality of documents. The function is in part defined by the customized ranking parameter.