As we have presented, it is evident that cohort studies are confronted with their own special, non-trivial issues of quality assurance and quality control. Such studies are typically large-scale designs and involve an extensive amount of data to be collected and processed, the quality of which depends on a variety of factors related to study personnel and equipment. The fact that data are collected over an extended period of time and at several centers greatly increases the magnitude of the data processing task, significantly increasing the likelihood of discrepancies and measurement error in the data. As presented in tables 1 and 2, the quality assurance and quality control procedures span the entire course of the study and include a multitude of tasks. Such tasks are delegated to various committees and/or are undertaken by participating centers, all of which must take responsibility for understanding, implementing, and following through on all procedures that maximize data quality. The quality of the quality assurance/quality control process is highly correlated with the quality of the communication within and between centers and all researchers. Maintaining standardization of procedures across centers and long-term stability of equipment and analytic procedures are integral components of quality control. In conclusion, the magnitude of the quality control process in a multicenter longitudinal study should not be underestimated, requiring a significant commitment of study resources. The quality control process is key to the integrity of the study, and an integral part of the design of the study. In a well-designed study, with a good quality control process and dedication to the process by the research team, the validity of the conclusions of the cohort study can be established.