Information Sources are categorized into 3 levels:

1.    Primary sources
Primary sources are raw data without interpretation that represents an official opinion. Examples of primary sources are memos, letters, interviews, laws, regulations, census, etc.

2.    Secondary sources
Secondary sources are interpretations of primary data. Examples of secondary data are encyclopedias, textbooks, handbooks, magazine articles, newspaper articles, etc.

3.    Tertiary sources
Tertiary sources may be interpretations of a secondary source, but generally represented by indexes, bibliographies, and other finding tools such as Internet search engines.