We are in an era of evidence-based medicine, with an exponentially increasing information base arising from descriptive and interventional studies. Clinicians are required to use this evidence to guide their clinical strategies for diagnosis and management of patients. This chapter discusses the basic principles of epidemiology and medical biostatistics.
TYPES OF EPIDEMIOLOGIC STUDIES
Epidemiologic studies are broadly divided into observational studies and experimental studies.1-6
Observational studies are based on naturally occurring phenomena and do not involve an intervention by the study team. There are several types:
Case reports and case series: These are descriptive studies that report the characteristics of the disease and frequency of events. They tend to report sentinel events that are useful in generating a hypothesis but cannot test them. They are useful for describing very rare diseases. In the hierarchy of the evidence pyramid, laboratory research and animal studies are at the very bottom followed by editorials, case reports, and case series.
Cross-sectional studies: These are analytic studies that evaluate a specified population for a given time. They may be used to determine the number of cases of a disease in the population at a particular point in time (ie, prevalence of a disease) and hence are also called prevalence studies. They are not suitable to evaluate incidence rates or to evaluate rare diseases or exposures. These are the weakest form of evidence among analytical observational studies, falling just above case series in the evidence pyramid.
Case-control studies: These are a form of analytic observational studies in which patients with a disease are selected for the case arm, and appropriate matched participants are selected for the control arm. These studies start with the disease and attempt to retrospectively identify causal associations. The advantages include that they are fast and inexpensive to conduct. They are well suited to study uncommon diseases and allow evaluation for multiple causal exposures. They are, however, subject to significant bias such as recall bias. Appropriate matching of control participants may be difficult. Case-control studies rank above cross-sectional studies but below cohort studies in the hierarchy of evidence.
Cohort studies: These are observational studies that follow a group of participants or a cohort with a known exposure as well as a cohort without the exposure, until development of the disease, to look for a difference in the incidence of the disease in the two groups. These studies may be prospective, retrospective, or mixed. They are suitable to estimate incidence, evaluate rare exposures, and evaluate for multiple outcomes. They can, however, be expensive and require long periods of follow-up.
Experimental studies involve an active controlled intervention such as an exposure or treatment.
Randomized controlled trials (RCTs): These are the most well-known experimental design. These are the best design to prove the relationship between exposure and event. Randomization and blinding are usually performed to minimize ...