| 1) Quality control starts with a well-prepared study, including use of valid, standardised methods, well-trained technicians and a thorough plan for data handling |
| 2) A pilot study should always be performed, testing the study team and all the methods used during the survey |
| 3) The pilot study should be a full-scale operation on a similar population as the one to be examined in the survey |
| 4) During data collection, quality control of each technician, all the instruments and questionnaires, and data handling should be performed regularly |
| 5) Quality control of the technicians implies checking the variability of their observations, and regular training and certification of each technician |
| 6) Quality control of the equipment implies regular calibration |
| 7) Quality control of the questionnaires implies regular control of uncompleted questions |
| 8) Quality control of data entry implies testing for outliers and inconsistent data (e.g. a never-smoker cannot report smoking 10 cigarettes per day) |
| 9) The results of quality control should be regularly assessed and discussed within the study team. This is particularly important in long studies to avoid fatigue among the technicians and reduced data quality |