A term of art for data collected for purposes other than for a formal study or trial.*
Because the investigator has no control over case definition, collection procedures, observers (clinicians) or other opportunities for measurement bias reduction, the potential for bias of secondary data is substantial.
*Secondary data has also been defined as data collected by a person who is not the user.
Examples Clinical records, which may be used for case-control studies; organisational records; consensus data.
Pros Cheaper; more readily available than primary data.
Cons Potential for significant measurement bias.