A secondary source is a document or recording that relates or discusses information originally presented elsewhere. It is one step removed from primary sources, though they often quote or otherwise use primary sources. Secondary sources can include most books about a topic, analysis or interpretation of data, scholarly or other articles about a topic, especially by people not directly involved, and documentaries. Secondary sources involve generalization, analysis, interpretation, or evaluation of the original information. They often attempt to describe or explain primary sources. Secondary sources may contain pictures, quotes, or graphics of primary sources. In some fields, a secondary source may include a summary of the literature in the introduction of a scientific paper, a description of what is known about a disease or treatment in a chapter in a reference book, or a synthesis written to review available literature. Whether something is a primary or secondary source often depends upon the topic and its use.