The data - sources and validation.

The basis for all observational studies is the availability of appropriate data of high quality. Data may be collected specifically for the research purpose in question (so-called "primary data"), but data collected for other purposes (so-called "secondary data") are also sometimes used and useful in research. High accuracy and precision are required (irrespective of the source of the data) to arrive at correct and unbiased results efficiently. Both careful planning prior to the start of the data acquisition and thorough procedures for data entry are obvious prerequisites to achieve high-quality data. However, data should also be subjected to a thorough validation after the collection. Primary data are mainly validated through proper screening, by using various descriptive statistical methods. Validation of secondary data is associated with specific conditions - the first of which is to be aware of the limitations in its usefulness imposed by procedures during collection. Approaches for validation of secondary data will be briefly discussed in the paper, and include patient chart review, combining with data from other sources, two-stage sampling, and aggregated methods.