The source for data is the medium through which data is received. Sources are categorized according to type and reliability, depending on how close they come to direct and reliable information. Because each source should be traced back as nearly as possible to its origin, sources which are closer to original data gathering are ranked higher than the sources they feed, and so on down the chain. Users of data categorized by these sources can thereby determine where their data needs further checking and refinement.

The immediate place where a fact is found can differ from the source that established it. A website quoting a document and a person relaying a measurement can both point to an earlier source that should be traced and cited where possible.

The source types retain different rules for interpretation and reliability. A direct measurement, a visual representation, and an unsourced estimate do not establish the same kind of evidence simply because they produce the same value.