The medical community is forced to accelerate the move from opinion- based to evidence-based medicine, that is, to aim at basing all caring and clinical practice on empiri. A clear-cut epistemology would facilitate this process. In this article we present a comprehensive clinical epidemiological theory which can be used for validity issues in caring science, quality of life research, controlled clinical trials and compilations of uncontrolled evidence. The theory is based on four distinct stages that can be identified in a study, whereof the first is demarcation of the source person-time. A source person-time ('study base') can be identified for any study in all disciplines, giving an argument for using this concept as the common reference point for validity issues. Apart from identifying the source person-time, recovery of the actually observed person-time, collection of data and calculation of an ('adjusted') effect parameter (e.g., incidence ratio) are additional stages of a study. When the source person-time is demarcated confounding is introduced, when the actually observed person-time is recovered misrepresentation, in the third stage misclassification and in the fourth analytical alteration of the parameter of effect. The concept of the source person-time can, in addition, link examination of validity in caring and clinical sciences to observational studies, thereby allowing the field to benefit from all theoretical achievements for preventing, handling and understanding systematic errors developed in epidemiology. We conclude it is possible to implement a common terminology of validity for all caring and medical sciences. Drawing causal inferences in these disciplines is not mechanical, it can never, for example, be done with statistical inference. Establishing a causal relation always includes an assessment of the magnitude and direction of systematic errors influencing the adjusted effect parameter. From the presented epistemology it follows that differences in validity from a case history to a large randomized, placebo-controlled and double-blinded study are quantitative rather than qualitative. This realization in turn opens up for a more refined discussion of when a decision is evidence-based rather than opinion-based.
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