Skilled writers can make you think their interpretations are facts. Assumptions should be reasonable. Note errors or omissions. The more radically an author departs from the views of others in the same field, the more carefully and critically you should scrutinize his or her ideas. You should explore enough sources to obtain a variety of viewpoints.
Primary sources are the raw material of the research process.
Secondary sources are based on primary sources. For example, if you were researching Konrad Adenauer's role in rebuilding West Germany after World War II, Adenauer's own writings would be one of many primary sources available on this topic. Others might include relevant government documents and contemporary German newspaper articles. Books, encyclopedia articles, and scholarly journal articles about Adenauer's role are considered secondary sources. In the sciences, journal articles and conference proceedings written by experimenters reporting the results of their research are primary documents.