Health statistics are sometimes derived from the records providers maintain to document the care that they provide. These case records serve as a source for information about episodes of illness and care and they can be used to derive indicators of the incidence of diseases, the response of providers and outcomes.
A leading benefit of these records is that they make it possible to track rare conditions that might not show up sufficiently in population surveys yet may constitute important factors in understanding health. While it is possible to derive estimates of mortality traced to these conditions from vital records [i.e. Death certificate abstracts] analyses based on case reports are key to measuring morbidity and the course of conditions that do not end in death.
The major limitation of these sources is that they depend on two key factors:
The extent to which persons with the condition “presents” i.e. comes to the attention of health care providers and
Classification and recording of these conditions by providers—i.e. whether and when they submit data as a case report.
Case reports are typically regarded as “passive surveillance” because they rely on providers to report cases rather than actively seeking reports.