It is important to use primary and secondary data to test researcher bias and to gather enough information to fully explore a topic. Primary research is any data that is gathered by the researcher. This includes observations, interviews and anything else that is learned during the data collection process. In historical research, primary data is first hand accounts of events and occurrences, and original documents.
Secondary data is any information that was gathered by other researchers. For example, census data, financial records and statistical information are considered secondary data. Most books and news reports are also considered secondary data.
There are faults with both types of research. First, primary research is subject to the researcher's interpretation of the data, which might not always be accurate. Second, the researcher might unknowingly bias the research by using a limited perspective. On the other hand, secondary data has to be verified by the researcher. Not all secondary data is credible, so using the wrong kind of secondary data can compromise the new research.
When primary and secondary data are used correctly, the researcher is able to test the validity of the research by analyzing secondary data. In addition, secondary data backs up the primary data the researcher collected.