Collection through medical records and registers are to be carefully carried out considering the period of recording, quality and applicability. This is truer when information about investigations or tests is gathered for the study as their methodology and significance may change with the time.

Study subjects may like the investigator or interested in the study and hence may give false or exaggerated information.

Collection of information about dose-response relationship is vital and unless it is collected meticulously, study looses its validity.