Historical control
A control group made up of persons who have been observed on an occasion previous to the present one involving observation of the group of interest.
Note: The validity of comparisons with this type of control group is suspect because the historical group is likely to differ from the current group in its composition, diagnosis, disease severity, measurement of outcomes, or other important ways that could influence the research findings. It may be feasible to use historical controls in special instances where the outcomes of a standard treatment (or no treatment) are well known and vary little for a given population.