Databases and registers: useful tools for research, no studies.

There are many misunderstandings about databases. Database is a commonly misused term in reference to any set of data entered into a computer. However, true databases serve a main purpose, organising data. They do so by establishing several layers of relationships; databases are hierarchical. Databases commonly organise data over different levels and over time, where time can be measured as the time between visits, or between treatments, or adverse events, etc. In this sense, medical databases are closely related to longitudinal observational studies, as databases allow the introduction of data on the same patient over time. Basically, we could establish four types of databases in medicine, depending on their purpose: (1) administrative databases, (2) clinical databases, (3) registers, and (4) study-oriented databases. But a database is a useful tool for a large variety of studies, not a type of study itself. Different types of databases serve very different purposes, and a clear understanding of the different research designs mentioned in this paper would prevent many of the databases we launch from being just a lot of work and very little science.