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In the development of questions for research or the use of published studies for evidence-based practice (EBP), we must be able to establish logical foundations for questions so that we can interpret findings. This is the essential interplay between theory and research, each integral to the other for advancing knowledge. Theories are created out of a need to organize and give meaning to a complex collection of individual facts and observations. The purpose of this chapter is to discuss components of theories, mechanisms for developing and testing clinical theories, and how we apply theory to research and the application of evidence.

Defining Theory

Scientific theory today deals with the empirical world of observation and experience, and requires constant verification. We use theory to generalize beyond a specific situation and to make predictions about what should happen in other similar situations. Without such explanations, we risk having to reinvent the wheel each time we are faced with a clinical problem.

A theory is a set of interrelated concepts, definitions, or propositions that specifies relationships among variables and represents a systematic view of specific phenomena.1

Research methods are the means by which we conduct investigations in a reliable and valid way so that we can observe clinical phenomena. But it is theory that lets us speculate on the questions of why and how things work, accounting for observed relationships. It allows us to name what we observe, provide potential explanations, and thereby figure out how we can change things in the future.


Theories have always been a part of human cultures, although not all have been scientific. Philosophy and religion have historically played a significant part in the acceptance of theory. The medieval view that the world was flat was born out of the theory that angels held up the four corners of the earth. Naturally, the men of the day were justified in believing that if one sailed toward the horizon, eventually one would fall off the edge of the earth. In healthcare we are aware of significant modifications to our understanding of the human body, as evidenced in the shift from Galen’s view of “pores” in the heart to Harvey’s theory of circulation. In the middle ages, medical theory was based on a balance among four “humours” (blood, black bile, yellow bile, and phlegm) and patients were bled and purged, made to vomit, and made to take snuff to correct imbalances. As theories change, so does our understanding of science and health.

Purposes of Theories

Theories can serve several purposes in science and clinical practice, depending on how we choose to use them.

  • Theories summarize existing knowledge to explain observable events—giving meaning to isolated empirical findings. They provide a framework for interpretation of observations. For example, a theory of ...

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