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The assessment of a health technology is frequently accompanied by uncertainty about its impact, at short or long terms, on the health of the population. The Health Authorities may request additional « post-registration » data that will allow a relevant reassessment of these technologies. The responsibility to collect this information lies with the industry and the HAS evaluates the methodology. This guideline provides practical benchmarks on methodological aspects of these studies. It describes the different types of studies to consider depending on the objectives, including the use of databases and cohorts and European studies. It emphasizes the importance of establishing a scientific committee, clearly defining the objectives of the study, justifying the methodological choices, documenting the representativeness or completeness of centers, investigators and patients, limiting the number of lost of follow-up patients and missing data, describing the statistical analysis methods, the bias and their possible impact on results. The publication of the results of these studies is strongly encouraged.