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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

What is primary research?

Primary research articles provide a report of individual, original research studies, which constitute the majority of articles published in peer-reviewed journals. All primary research studies are conducted according to a specified methodology, which will be partly determined by the aims and objectives of the research.

The following sections offer brief summaries of some of the common quantitative, qualitative, and mixed-methods study designs you may encounter. 

Quantitative research study designs

Randomised Controlled Trial

A randomised controlled trial (RCT) is a study where participants are randomly allocated to two or more groups. One group receives the treatment that is being tested by the study (treatment or experimental group), and the other group(s) receive an alternative, which is often the current standard treatment or a placebo (control or comparison group). The nature of the control used should always be specified.

An RCT is a good study choice for determining the effectiveness of an intervention or treatment, or for comparing the relative effectiveness of different interventions or treatments. If well implemented, the randomisation of participants in RCTs should ensure that the groups differ only in their exposure to treatment, and that differences in outcomes between the groups are probably attributable to the treatment being studied.

In crossover randomised controlled trials, participants receive all of the treatments and controls being tested in a random order. This means that participants receive one treatment, the effect of which is measured, and then "cross over" into the other treatment group, where the effect of the second treatment (or control) is measured.

RCTs are generally considered to be the most rigorous experimental study design, as the randomisation of participants helps to minimise confounding and other sources of bias.


Cohort study

A cohort study identifies a group of people and follows them over a period of time to see who develops the outcome of interest to the study. This type of study is normally used to look at the effect of suspected risk factors that cannot be controlled experimentally – for example, the effect of smoking on lung cancer.

Also sometimes called longitudinal studies, cohort studies can be either prospective, that is, exposure factors are identified at the beginning of a study and the study population is followed into the future, or retrospective, that is, medical records for the study population are used to identify past exposure factors.

Cohort studies are useful in answering questions about disease causation or progression, or studying the effects of harmful exposures.

Cohort studies are generally considered to be the most reliable observational study design. They are not as reliable as RCTs, as the study groups may differ in ways other than the variable being studied.

Other problems with cohort studies are that they require a large sample size, are inefficient for rare outcomes, and can take long periods of time.


Case-Control Study

A case-control study compares a group of people with a disease or condition, against a control population without the disease or condition, in order to investigate the causes of particular outcomes. The study looks back at the two groups over time to see which risk factors for the disease or condition they have been exposed to.

Case-control studies can be useful in identifying which risk factors may predict a disease, or how a disease progresses over time. They can be especially useful for investigating the causes of rare outcomes.

Case-control studies can be done quickly, and do not require large groups of subjects. However, their reliance on retrospective data which may be incomplete or unreliable (owing to subject ability to accurately recall information such as the appearance of a symptom) can be a difficulty.


Cross-Sectional Study

A cross-sectional study collects data from the study population at one point in time, and considers the relationships between characteristics. Also  sometimes called surveys or prevalence studies.

Cross-sectional studies are generally used to study the prevalence of a risk factor, disease or outcome in a chosen population.

Because cross-sectional studies do not look at trends or changes over time, they cannot establish cause and effect between exposures and outcomes.


Case Series / Case Reports

A case series is a descriptive study of a group of people, who have either received the same treatment or have the same disease, in order to identify characteristics or outcomes in a particular group of people.

Case series are useful for studying rare diseases or adverse outcomes, for illustrating particular aspects of a condition, identifying treatment approaches, and for generating hypotheses for further study.

A case report provides a study of an individual, rather than a group.

Case series and case reports have no comparative control groups, and are prone to bias and chance association.


Expert opinion

Expert opinion draws upon the clinical experience and recommendations of those with established expertise on a topic.

Qualitative research study designs

Grounded theory

Grounded theory studies aim to generate theory in order to explain social processes, interactions or issues. This explanatory theory is grounded in, and generated from, the research participant data collected.

Research data typically takes the form of interviews, observations or documents. Data is analysed as it is collected, and is coded and organised into categories which inform the further collection of data, and the construction of theory. This cycle helps to refine the theory, which evolves as more data is gathered.



A phenomenological study aims to describe the meaning(s) of the lived experience of a phenomenon. Research participants will have some common experience of the phenomenon under examination, but will differ in their precise individual experience, and in other personal or social characteristics.

Research data is typically in the form of observations, interviews or written records, and its analysis sets out to identify common themes in the participants' experience, while also highlighting variations and unique themes.



Ethnography is the study of a specific culture or cultural group, where the researcher seeks an insider perspective by placing themselves as a participant observer within the group under study.

Data is typically formed of observations, interviews and conversation. Ethnography aims to offer direct insight into the lives and the experiences of the group or the culture under study, examining its beliefs, values, practices and behaviours.

Case study

A case study offers a detailed description of the experience of an individual, a family, a community or an organisation, often with the aim of highlighting a particular issue. Research data may include documents, interviews and observations.


Content analysis

Content analysis is used to explore the occurrence, meanings and relationships of words, themes or concepts within a set of textual data. Research data might be drawn from any type of written document(s). Data is coded and categorised, with the aim of revealing and examining the patterns and the intentions of language use within the data set.


Narrative inquiry

A narrative inquiry offers in depth detail of a situation or experience from the perspective of an individual or small groups. Research data usually consists of interviews or recordings, which is presented as a structured, chronological narrative. Narrative inquiry studies often seek to give voice to individuals or populations whose perspective is less well established, or not commonly sought.


Action research

Action research is a form of research, commonly used with groups, where the participants take a more active, collaborative role in producing the research. Studies incorporate the lived experiences of the individuals, groups or communities under study, drawing on data which might include observation, interviews, questionnaires or workshops.

Action research is generally aimed at changing or improving a particular context, or a specific practice, alongside the generation of theory.

Mixed methods research study designs


Explanatory sequential design

In an explanatory sequential study, emphasis is given to the collection and analysis of quantitative data, which occurs during the first phase of the study. The results of this quantitative phase inform the subsequent collection of qualitative data in the next phase.

Analysis of the resultant qualitative data is then used to 'explain' the quantitative results, usually serving to contextualise these, or to otherwise enhance or enrich the initial findings.


Exploratory sequential design

In an exploratory sequential study, the opposite sequence to that outlined above is used. In this case, qualitative data is emphasised, with this being collected and analysed during the first phase of the study. The results of this qualitative phase inform the subsequent collection of quantitative data in the next phase.

The quantitative data can then be used to define or to generalise the qualitative results, or to test these results on the basis of theory emerging from the initial findings.


Convergent design

In a convergent study, qualitative and quantitative data sets are collected and analysed simultaneously and independently of one another.

Results from analysis of both sets of data are brought together to provide one overall interpretation; this combination of data types can be handled in various ways, but the objective is always to provide a fuller understanding of the phenomena under study. Equal emphasis is given to both qualitative and quantitative data in a convergent study.