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Understanding research and critical appraisal

This guide provides an outline of the key types of research study designs, and the difference between primary and secondary research

Purpose of this guide

This guide offers an overview of medical and health-related research - including a brief summary of research approaches - and outlines the key features of some of the most common research study designs. It also provides a hierarchy of evidence, showing the relative levels of evidence these study types are commonly held to represent, based upon features of their design which make them more or less robust.

The need for critical appraisal of research articles is also discussed. The guide highlights some basic questions that can help with understanding and evaluating a paper, and suggests some additional resources to support more detailed critical appraisal.

Finally, the guide also includes a glossary of terminology that you may encounter when reading research papers.

Research approaches

When designing a research study, one of three different research approaches might be employed; these being qualitative, quantitative or mixed-methods.

To some extent, the nature of the question being studied will determine both the appropriate research approach to be taken, and the most suitable study design, though additional philosophical, ethical or practical factors may also influence the decision. The below offers a broad, general description of each approach.


Quantitative research

Quantitative research aims to examine relationships between variables (e.g. persons, phenomena), in order to test hypotheses about their relationship, or to explain phenomena in a quantitative way. Examples of quantitative research could include examining the health effects of a lifestyle factor, or measuring the effects of a particular medical intervention on a health condition.

Quantitative studies recruit a sample group of research participants and collect numerical data from this sample population - the participants should be representative of the broader population, in order for the study findings to be generalisable. The same data is recorded and measured in the same way for all participants and is analysed, often statistically, to offer explanations of the phenomena being studied.


Qualitative research

Qualitative research focuses on exploring the meaning(s) that individuals or groups attribute to experiences or phenomena, or their beliefs or attitudes about or towards experiences.

The focus of study is typically real world situations and settings, with an attempt to capture rich, detailed data from research participants, often through interview, observation, or the analysis of existing documents. Qualitative studies attempt to explain meaning, or to construct themes or theory from the data collected, rather than setting these out beforehand.


Mixed-methods research

In mixed-methods research, both quantitative and qualitative data are collected, and integrated according to specific approaches, with the assumption that combining both forms of data can provide a fuller explanation or understanding of the phenomena being studied.

Levels of research evidence

Generally, when using quantitative research, you should aim to find studies from the strongest level of the below hierarchy, bearing in mind that the nature of the topic being investigated can determine which type of studies might be conducted. There are many cases, for example, where it would not be ethical to conduct a randomised controlled trial.

The hierarchy does not include qualitative research studies, but in cases where a question relates to examining experience(s), attitude(s) or belief(s), qualitative research studies will provide the most appropriate evidence.

It is always important to bear in mind that research studies can vary in quality, and some appraisal of research articles is helpful, regardless of the type of study. Do not assume that one type of study design is always better than another type: instead, it is important to consider and evaluate the merits of individual studies. See the section on critical appraisal for more detail.


A diagram on Types of Scientific Evidence; for more detail follow the link below.































"A Rough Guide to Types of Scientific Evidence" by Compound Interest is licensed under CC BY-NC-ND 4.0