Different research designs
What is research?
Research can be difficult to define, and this is reflected in the definition provided by the Australian code for the responsible conduct of research (National Health and Medical Research Council, Australian Research Council, & Universities Australia 2007):
“original investigation undertaken to gain knowledge, understanding and insight. It is a broad concept and there is no simple, single way to define research for all disciplines.”
Before starting any research, it is important to determine the most appropriate research and study type to answer your question. “In contrast to errors in the statistical evaluation, errors in design cannot be corrected after the study has been completed”. (Rohig 2009)
In this guide we draw on a number of published reviews and resources to provide a general introduction to some of the research and study types relevant to research in health care. For more information, we encourage interested readers to explore the references and the list of other useful resources at the end of this guide.
Research design and types
Research Study Design - The first step in research is to define your question, and this is part of the study design or overall plan for the study. The question will determine the most appropriate research and study types to use.
- Question to be answered –Why? Who? What? When? Where? How many? The use of frameworks such as PICO can be very useful in defining the research question. Note however, that for some questions only part of the framework may be relevant.
- Patient (or population)
To determine the most appropriate research and study type to answer your question, it is then useful to consider some of the other research design components to help define the project and ensure that the study conclusions are relevant and reliable:
- Study population – is the research sample or population representative?
- Type of study – What type of study is most appropriate to answer the question? (see below)
- Unit of observation – Will the research target a selected patient group, population, cell, organ or organ system?
- Measuring technique– When will the measurements be taken and how often? Who will take the measurements?
- Calculation of sample size–How many subjects/data points should be included to obtain a meaningful result?
Research types and important terms - In health and medical research there are two main types of research; primary and secondary, see Figure 1. Both primary and secondary research are divided into a number of specific areas of research, and depending on that area a number of different study types are available to address the research question of interest. Choosing the most appropriate study type for your question begins with understanding the different research and study types available, and being familiar with the relevant terms.
Primary research - actual data are collected. In primary health research, there are generally four major areas of research as shown in Figure 1 below; laboratory, clinical, clinical epidemiology, and epidemiological research.
Secondary research – this refers to a ‘study of studies’, aimed at summarising available studies in the form of reviews. In secondary research, reviews incorporating statistical analysis to integrate outcomes from different studies are distinguished from reviews not employing this level of analysis.
Distinction is also made between qualitative and quantitative research types:
Qualitative studies – studies that describe attitudes and behaviour without collection of numerical data.
Quantitative studies – studies that generate numerical data or data that can be converted into numbers, e.g.case report, case series, RCT, Cohort study, systematic review, economic study.
Mixed methods studies - Where both qualitative and quantitative research types are combined in a body of investigation, e.g. health care surveys or program evaluation studies.
Figure 1 Different research types
Primary research study types
Primary research involves data collection, and draws on a range of study types depending on a number of factors, including the question, area of research, population, disease, outcome of interest, time available, and ethical considerations. For clinical, clinical epidemiological and epidemiological research, these study types will be either:
- Interventional (experimental) – where participants receive specific interventions according to a pre-specified study protocol. This includes controlled and uncontrolled trials, and randomised controlled trials. Bias is minimised by randomisation and blinding.
- Non-interventional (observational) –investigators simply observe the course of events without action by the investigator. This includes monitoring, surveillance, cross-sectional study, secondary data analysis and retrospective data collection.
Primary study types - A selection of some of the more common primary study types employed in health research are listed below, as defined in the Cochrane Library Community (beta) Glossary and/or the National Institute for Health and Care Excellence (NICE) glossary of study designs. Click the symbol, to view an example study or more information about the study type.
- Case report
- Case series
“A study reporting observations on a series of individuals, usually all receiving the same intervention, with no control group.” For example, adverse events, or unexpected therapeutic effect in patients attending a single clinic.
- Case control study
“A study that compares people with a specific disease or outcome of interest (cases) to people from the same population without that disease or outcome (controls), and which seeks to find associations between the outcome and prior exposure to particular risk factors. This design is particularly useful where the outcome is rare and past exposure can be reliably measured. Case-control studies are usually retrospective, but not always.” For example, investigation of cancer.
- Cluster randomised controlled trial
“A trial in which clusters of individuals (e.g. clinics, families, geographical areas), rather than individuals themselves, are randomised to different arms. In such studies, care should be taken to avoid unit of analysis errors.” For example, investigation of school based preventive health programs.
- Cohort study
“An observational study in which a defined group of people (the cohort) is followed over time. The outcomes of people in subsets of this cohort are compared, to examine people who were exposed or not exposed (or exposed at different levels) to a particular intervention or other factor of interest. A prospective cohort study assembles participants and follows them into the future. A retrospective (or historical) cohort study identifies subjects from past records and follows them from the time of those records to the present. Because subjects are not allocated by the investigator to different interventions or other exposures, adjusted analysis is usually required to minimise the influence of other factors (confounders).” Also often called a longitudinal study. For example, estimating incidence rates in exposed populations, or mortality risk relative to several potential causes.
- Cross-sectional study (survey)
- Interrupted time series
“A research design that collects observations at multiple time points before and after an intervention(interruption). The design attempts to detect whether the intervention has had an effect significantly greater than the underlying trend.”
- Interview study
“A qualitative method of data collection where participant's views are elicited via verbal interviews. Interviews can be structured (that is, reliant on set questions), semi-structured (the interviewer has a general idea of topics and themes to cover and will steer the interview towards these participants) or unstructured (the format and direction of the interview is set by the participant). In addition, interviews can be 1-to-1, or with couples or small groups (see focus groups). They can be conducted in person, on the telephone, or more recently online. Interviews are usually recorded and transcribed and analysis is undertaken on the transcripts rather than the verbal data.” For example, health service delivery evaluation study.
- Observational study
“A study in which the investigators do not seek to intervene, and simply observe the course of events. Changes or differences in one characteristic (e.g. whether or not people received the intervention of interest) are studied in relation to changes or differences in other characteristic(s) (e.g. whether or not they died), without action by the investigator. There is a greater risk of selection bias than in experimental studies.” For example, study of uptake of self-management tools such as glucose monitors.
- Randomised controlled trial
“An experiment in which two or more interventions, possibly including a control intervention or no intervention, are compared by being randomly allocated to participants. In most trials one intervention is assigned to each individual but sometimes assignment is to define groups of individuals (for example, in a household) or interventions are assigned within individuals (for example, in different orders or to different parts of the body).” For example, investigation of the efficacy of a specific treatment.
- Retrospective study
“A study in which the outcomes have occurred to the participants before the study commenced. Case-control studies are usually retrospective, cohort studies sometimes are, randomised controlled trials never are.”
Secondary research study types
Secondary research is based on studying other studies and this can take the form of a meta-analysis or a review. But not all reviews are the same; the way in which the other studies are identified and selected affects the rigour of the review.
The most appropriate review type will depend on the question being addressed, the availability of other studies, the time available and the intended use of the outcomes. Some examples of review types are defined in the table below.
Choosing the most appropriate study type
Choosing the most appropriate study type will depend on the research question being asked and answers to the questions framed above, i.e.
- Primary or secondary research?
- Quantitative or qualitative research, or both?
- Interventional or non-interventional research?
- What is the intervention or outcome of interest?
- What is the population of interest?
The answers to these questions can then be combined with an understanding of the established study types available to decide on the most appropriate study type for the research question being investigated.
Other useful resources that may guide your choice of study type include:
- Published literature reporting similar research and the study type that was used.
- Published guides/decision trees, a selection of these is listed below:
- National Health and Medical Research Council (NHMRC). How to use the evidence: assessment and application of scientific evidence (2000), pp1-2. Available at https://www.nhmrc.gov.au/guidelines-publications/cp69 last accessed 22nd April 2015. Study type suggestions based on the type of question.
- Muir Gray Evidence-Based Health Care and Public Health: How to Make Decisions About Health Services and Public Health, 2014, chapter 5. A matrix table of the most appropriate study type based on the intervention or outcome of interest.
- The University of Oxford centre for Evidence Based Medicine provides a useful decision tree guide for selecting the most appropriate primary study type based on study Population, Outcomes, and Intervention or Exposure.
- For epidemiological studies, Blettner et al 2001 provides a useful review with a list of study types well suited to selected epidemiological study objectives.
Research evidence hierarchy
The wide range of study types in research has led to the ‘evidence hierarchy’ where the potential for bias in outcomes reported from a particular study type is used to define its ranking (evidence level) compared to other study types. On this basis those study types designed to minimise bias are ranked higher. There is no single evidence hierarchy, but most systems place randomised controlled trials near the top, whilst acknowledging that this is not always the most appropriate study type.
Grading of evidence generally begins with evidence level, but then incorporates other factors including consistency, generalisability, and applicability of the evidence. Depending on the list of contributing factors considered the final grading may differ. (Luoto 2013)
- Evidence level is just one part of the NHMRC evidence grading system
- The NHMRC evidence hierarchy for primary intervention studies is shown below. The NHMRC evidence hierarchy for screening interventions is very similar to this, but it should be noted that the NHMRC evidence hierarchies for questions related to diagnostic accuracy, prognosis, or aetiology differ for levels II through to IV.
Besen J, Gan SD. (2014). A critical evaluation of clinical research study designs. J Investigative Dermatology, 134, e18.
Blettner M, et al. (2001). Critical reading of epidemiological papers. Eur J Public Health, 11(1), 97-101.
Cochrane collection Cochrane Library Community (beta) Glossary. Retrieved April 2015 from http://community.cochrane.org/glossary/5.
Curry LA, et al. (2013). Mixed Methods in biomedical and Health Services Research. Circ Cardiovasc Qual Outcomes, 6, 119-123.
Khangura S, et al. (2012). Evidence summaries: the evolution of a rapid review approach. Syst Rev, 1, 10.
Luoto J, et al. (2013). A Comparison of Frameworks Evaluating Evidence for Global Health Interventions. PLoS Med 10(7), e1001469.
National Institute for Health and Care Excellence (NICE) Glossary of study designs. Retrieved April 2015 from http://www.nice.org.uk/article/pmg4/chapter/appendix-d-glossary-of-study-designs.
NHMRC additional levels of evidence and grades for recommendations for developers of guidelines 2009. Retrieved April 2015 from
Pawson R, et al. (2005). Realist review – a new method of systematic review designed for complex policy interventions. J Health Serv Res Policy, 10(suppl 1), 21-34.
Ressing M, et al. (2009). Systematic literature reviews and meta-analyses: part 6 of a series on evaluation of scientific publications. Dtsch Arztebl Int, 106(27), 456-63.
Röhrig B, et al. (2009). Study design in medical research: part 2 of a series on the evaluation of scientific publications. Dtsch Arztebl Int, 106(11), 184-9.
Röhrig B, et al. (2009b). Types of study in medical research: part 3 of a series on evaluation of scientific publications. Dtsch Arztebl Int, 106(15), 262-8.
Srikanth and Doddamani. (2013). Overview of study designs. Int J Pharm Pharm Sci, 5(3), 1020-24.
Useful PHCRIS resources
PHCRIS Research Profiles
(formerly ROAR - Register Of Australian primary health care Research) is Australia's only online platform for anyone involved in primary health care research. Explore and share career achievements, research ideas, as well as ongoing and future projects.
Fact sheets about research compiled by the University of New South Wales as part of the NSW Primary Health Care Research Capacity Building Program.
Research methods appropriate to public health practice, presented by the Health Knowledge group supported by the UK Department of Health. Retrieved April 2015 from http://www.healthknowledge.org.uk/public-health-textbook/research-methods
Study Designs, presented by the Centre for Evidence-Based Medicine (CEBM), University of Oxford, UK. Retrieved April 2015 from http://www.cebm.net/study-designs/
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Primary Health Care Research & Information Service (2017). PHCRIS Getting Started Guides: Introduction to... Different research models. From http://www.phcris.org.au/guides/about_research.php (Accessed 27 Jul 2017)