There are many types of studies used in biomedical research. Knowing these types, their strengths and weaknesses and what questions they are most suited to answering, helps in determining their relevance to your question.
Primary studies are reports of original research. Primary studies may be interventional (also known as experimental) or non-interventional (observational).
Interventional studies compare the effects of an intervention against some kind of control. Randomisation and blinding are used to increase the strength of the results. Experimental studies can infer cause and effect but may not reflect 'real world' conditions.
Uses a control and an experimental group to which participants are randomly assigned, with a comparison made at the end of the study. RCTs are used for intervention/therapy studies.
Advantages: assessing causality, and clearly demonstrating that the intervention caused the results.
Disadvantages: expense, time-consumption, external validity (applying the results to populations or settings different to the study group), and risk of bias if participants are not properly blinded. May not pick up adverse effects.
Non-interventional (or observational) studies investigate the occurrence, distribution and progression of conditions. They are descriptive rather than analytical, but better reflect 'real life' situations.
Groups of individuals are followed over time to assess the rate of occurrence of a disease and identify risk factors.. Cohort studies answer diagnostic test accuracy questions, etiology/risk questions for common outcomes resulting from unusual exposure and longitudinal cohorts for prognosis questions. Cohort studies can be retrospective or prospective.
Advantages: Researchers can identify relative risk of developing a disease based on different exposures.
Disadvantages: time and cost.
Patients with a condition are matched with controls without the condition. These studies begin with the outcomes. The cases are reviewed to identify past exposures to risk which are then compared with the control group. They compare the odds of having an experience with the outcome to the odds of having an experience without the outcome. Case-control studies are good for answering etiology/ risk questions where a rare outcome resulted from a common exposure.
Reviews of single cases. Case studies allow detailed examination of complex or unusual problems.
Advantages: Holistic view of phenomena, insight into rare/unusual/unstudied presentations, identify novel issues.
Disadvantages: not generalisable, risk of bias.
BMJ Evidence Based Nursing. What is a case study?
Data collected at a defined time. Provide a snapshot of health status. These studies are useful for prevalence questions, establishing baseline data and population need.
Advantages: data assessed may have already been collected for other purposes.
Disadvantages: recall bias if using interviews. They cannot be used for rare diseases.
Secondary studies do not involve direct research. Instead they analyse research that has already been done and try to summarise the results of multiple primary studies. By looking at multiple pieces of research, secondary studies can have greater statistical power than primary research.
A literature review that asks a focused question/s and uses explicit, systematic methods to identify, appraise, select and synthesise all high quality research evidence relevant to that question/s. Some include a meta analysis (definition below). Systematic reviews are good for answering intervention/therapy questions. The Cochrane Library produces high-quality systematic reviews. You can also find systematic reviews in PubMed Clinical Queries. The Cochrane Library also contains systematic reviews written by organisations other than Cochrane.
Advantages: Summarises and analyses multiple research studies. Should describe strengths and weaknesses of evidence.
Disadvantages: May be addressing very specific question.
See our guide on systematic reviews for more information. (under construction)
A systematic review that extracts, summarizes and analyses the data from multiple studies using sophisticated statistical methods. Some statisticians rank these as having greater strength than systematic reviews.
BMJ Evidence Based Nursing. What is meta-analysis?
Study Designs in Medicine - PubMed
Types of Study in Medical Research - PubMed