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INTRODUCTION

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Throughout this text, we have addressed the importance of understanding basic elements of research design and statistics for clinical decision making, especially within the context of evidence-based practice. In this chapter we will present an important perspective in health care research based on principles of epidemiology. The information from epidemiological research can have direct influence on practitioners' day-to-day choices related to diagnosis, prognosis or intervention. The purpose of this chapter is to present statistical methods for measures of disease frequency, estimates of health risks for cohort and case-control studies, and the evaluation of treatment effects in randomized trials.

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THE SCOPE OF EPIDEMIOLOGY

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Classically, the field of epidemiology is concerned with the study of the distribution and determinants of disease, injury, or dysfunction in human populations. Epidemiology literally began as the study of "epidemics," concerned primarily with mortality and morbidity from acute infectious diseases. Many of the health standards we take for granted today, such as clean water supplies, treatment of sewage and food refrigeration, can be credited to discoveries made through epidemiological investigations.

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Epidemiologists try to identify those who have a specific disorder, when and where the disorder developed and what exposures are associated with its presence. Epidemiological questions often arise out of clinical experience, laboratory findings or public health concerns about the relationship between societal practices and disease outcomes. Through the analysis of health status indicators and population characteristics, epidemiologists try to identify and explain the causal factors in disease patterns.

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As medical cures and treatments have been developed to control many of these problems, and as patterns of disease have changed, the scope of epidemiology has broadened. Today epidemiology includes the study of chronic disease, disability and health status. Because we are often concerned with functional problems as well as disease states, we will use the terms disease, disorder and disability interchangeably to represent health outcomes, including illness, injury and physical, psychological or social dysfunction. This approach fits with the World Health Organization's definition of health, which encompasses social, psychological and physical well-being.1

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Epidemiology is distinguished as a research approach because of its unique concern with the identification of risk factors for disability and disease. Epidemiologic studies are generally distinguished as observational or experimental (or quasi-experimental). In observational studies there is no artificial manipulation of any of the study factors (see Chapter 13). Observational studies are categorized as descriptive or analytic. Descriptive studies are concerned with the distribution and patterns of disease or disability in a population. These are carried out when there is little knowledge about the state of health or frequency of disease. Analytic studies test hypotheses to determine if specific exposures are related to health status or disease occurrence. Case-control and cohort studies are observational analytic approaches (see Chapter 13). Randomized controlled trials (RCT) are experimental analytic studies that are designed to test the effect of interventions on ...

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