As part of the Constances cohort, the 200,000 participants are invited to visit one of the 22 participating health screening centers to undergo an initial examination that includes, among other things, paraclinical measurements and laboratory tests. These centers, equipped with a variety of brands and technologies, rely on numerous staff members to collect data. These conditions, which lead to significant variability between operators and centers, are inherent to cohorts of this size. From the outset, significant resources were therefore invested in establishing a robust quality system. The protocol and standard operating procedures (SOPs) were developed in collaboration with center staff and experts from various specialties. The SOPs define the minimum equipment specifications and precisely describe each step of the measurement process to prevent pre-identified biases. Operator training and monitoring of SOP compliance are conducted through monthly on-site monitoring. Additionally, automated database programs detect potential inconsistencies or deviations. In addition to these measures, the biology department benefits from the results of monitoring by a certified external organization shared by all biological analysis laboratories. Constances can thus offer researchers, in addition to the data collected from participants, a wealth of information enabling them to assess the level of confidence to be placed in each data point, as well as the need to perform adjustments or censoring, so as not to alter the conclusions of their work.