A study that compares retrospectively people with a specific disease or outcome of interest (cases) to people from the same population without that disease or outcome (controls), and which seeks to find associations between the outcome and prior exposure to a particular factor of interest (e.g., a micronutrient). 

Due to the potential for many forms of bias (e.g., when selecting cases and controls or recalling nutrition behaviors) in this study type, case control studies provide relatively weak empirical evidence even when properly executed.