As a part of the service they provide to patients, medical doctors, nurses and allied health workers compile notes that document the conditions they encounter, the treatments provided and the outcomes of those treatments. These clinical observations can also be summarized in statistical tables. Taken together they show emerging trends and indicate areas that need to be more systematically documented.
One advantage of this approach is that it is based on the observations of the most knowledgeable and highly trained health workers.
As a result it provides information about complicated and subtle effects and can be related to actual diagnoses that require the observers to invoke a high degree of medical intuition as well as appropriate laboratory tests.
One disadvantage is that health providers only see those who present in their service. As a result their records are limited to people who receive care from them and do not reflect the conditions of a defined population.
Another disadvantage is that these records only record conditions that are identified by the medical system—and medical service does not require that each patient would be measured with the same battery of tests and observations.
This means that generalizations are not based on statistical procedure but rely on other types of information—such as inferences about the causes of conditions.