Neither better nor worse, simply different.

Have you ever suffered discrimination because you used secondary data in your research? Since the principal area of research for this article’s three authors involves the development and application of techniques for using secondary data, our answer is definitely no. However, we frequently hear complaints by colleagues who have encountered barriers to developing their theses or obtaining research funding because they opted to use secondary data. A recent article by Rothman 1 discusses six erroneous perceptions regarding aspects of epidemiological research that are often reinforced in classrooms and textbooks. Although the author did not discuss data sources, we believe that the list should add a seventh misconception: the notion that primary data are the only valid source for epidemiological studies. Population, vital, epidemiological, administrative, and clinical data have undergone important changes in their production and dissemination. They are now available in online databases that include millions of individual micro-data. In addition to the above-mentioned traditional sources, other modalities have emerged. The digital trails produced in accessing different web-based communication platforms and mobile phones have been used in studies about how patterns of behavior and mobility influence the determination and spread of diseases 2. Secondary data have the potential to back studies on highly relevant public health issues, particularly due to their wide availability, scope, and coverage. They are actually the best data to answer questions on the determinants of incidence rates in populations, as suggested by Rose 3. Even so, it is important to discuss how the two worlds are brought together. For example, geneenvironment interaction requires the use of increasingly larger study populations. The context of “big epidemiology” 4 stimulates the practice of “data sharing”, whereby the data collected for specific studies are used by researchers not originally involved in their planning and execution. The age of “big data” has brought about the recommendation of using this wealth of data in research 5, including population health research 6. However, several authors have emphasized the need for responsible use of such databases 7. The main criticisms aimed at secondary data sources are the absence of mechanisms for data quality assurance and control and the lack of necessary variables for adequately testing causal hypotheses at the individual level. Quality is a crucial issue. One should evaluate the different dimensions of quality 8 before using a secondary data source. Meanwhile, database custodians should employ techniques to prevent, detect, and repair errors 9 and make extensive documentation available on their data 1363 PERSPECTIVAS PERSPECTIVES