Primary and secondary data are defined by their method of research. Primary data is both researched and used by the same source. On the other hand, secondary data comes from a synthesis of external sources' research, such as an analysis. A common form of primary data is published government statistics, while secondary data is often found in textbooks and online articles.
Experiments and surveys are common ways to acquire primary data. Scientists often publish their findings in medical journals for public reference. Nielsen (nielsen.com) is a public survey company which measures the number of media viewers -- television, radio and mobile. Secondary data is usually created by experts who interpret primary data. They form logical opinions, predict future trends and advise solutions based on primary data. This secondary data is then used by the general public to gather information or to create more secondary data. Information portals such as Yahoo! (yahoo.com) and research databases such as LexisNexis (lexisnexis.com) typically contain secondary data.
A major issue in primary and secondary data is the credibility of the data. David L. Kurtz, author of "Contemporary Marketing" states that secondary data is prone to more quickly become obsolete. A report on an event and the conclusions drawn in secondary data is time sensitive; another event which disproves prior conclusions can occur. David L. Kurtz states that the content determines the credibility of the data. For example, secondary data is suitable to use in a study of long-standing societal issues while primary data is more credible for a news report.
Applying primary data is typically more complex than secondary data. According to David L. Kurtz, utilizing primary data almost always takes longer and is more expensive than secondary data; acquiring and compiling primary data can take up to three to four months while secondary data can be acquired within minutes for almost free. Researchers typically apply both types of data to their studies to completely answer questions comprehensively and efficiently.