Chapter 3: Types of Studies in Clinical Research—Part I: Observational Studies

INTRODUCTION

The previous chapter described the various steps of planning and conducting a research study. This chapter briefly introduces the reader to the different types of studies and then elaborates on the observational studies. In observational studies, the researcher observes the involvement of the participants and collects data by simply observing events as they happen, without playing an active part in what takes place. In interventional, or experimental, studies, the investigator exposes the participants to some kind of intervention and tries to find a relation between the intervention and the outcome. Observational studies can be descriptive, like the case studies and case series, but are more commonly analytical (cross-sectional, case–control, and cohort studies). Descriptive observational studies describe characteristics of a population and usually do not have a hypothesis; they are sometimes hypothesis-generating studies. An analytical observational study, in addition, tries to find a causal relationship between two or more comparable groups (variables) and has a hypothesis to prove.

RESEARCH DESIGN AND STUDIES

A study design is a road map or blueprint based on the type of research to be carried out. It starts with development of the research question, formulating a hypothesis and research objectives, and subsequent planning for carrying out the research. The research objectives of the proposed study determine the type of study to be undertaken.

Types of Studies

Type of studies in medical research can be broadly classified into primary and secondary studies. Primary studies are those that are actually performed by the investigators, while secondary studies summarize the results of different primary studies in the form of systematic reviews and meta-analyses without actually performing the studies.¹ Primary studies can be put into three groups based on the type of research undertaken: basic medical or experimental studies, epidemiologic studies, and clinical studies. Basic medical studies include research in animal experiments, cell studies, biochemical, genetic and physiologic investigations, and studies on the properties of drugs and materials. Epidemiologic studies investigate the distribution and historical changes in the frequency of diseases and the causes for these diseases, while clinical studies involve research in human subjects. However, it may be difficult to classify individual studies into one of these three main categories.¹ A more practical way to classify the types of research studies based on their function is to group them into observational and interventional (experimental) studies; the former can be further subclassified into descriptive and analytical studies (Figure 3-1).