Which type of research study provides the best evidence in a evidence report?

Using the best research evidence is important. However, sometimes the current evidence is inconclusive, or of a low quality.  

Types and levels of evidence 

To inform your clinical practice, it’s best to look for the highest level of evidence available. A pyramid is often used to represent the hierarchy of evidence, with the higher quality evidence at the top.  

Best evidence includes empirical evidence from randomized controlled trials; evidence from other scientific methods such as descriptive and qualitative research; as well as use of information from case reports, scientific principles, and expert opinion.

Source: Patient Safety and Quality: An Evidence-Based Handbook for Nurses. Chapter 7. 

For example, the following evidence pyramid provides a hierarchy of research and study types:

The levels of evidence (highest to lowest) are:

1. Systematic reviews and meta-analyses
2. Evidence summaries and guidelines
3. Randomised controlled trials
4. Cohort studies, case control studies, case series and case reports
5. Background information, expert opinions and editorials.

Systematic reviews and meta-analyses 

Systematic reviews aim to identify, evaluate and summarise the findings of all relevant individual studies, thereby making the available evidence more accessible to decision-makers. When appropriate, combining the results of several studies gives a more reliable and precise estimate of an intervention’s effectiveness than one study alone.

Source: Systematic Reviews: CRD's guidance for undertaking reviews in health care.

Dr Benjamin Spock’s advice on babies’ sleeping position was disproved according to the evidence from studies and a systematic review. A systematic review published in 2005 concluded that:

Advice to put infants to sleep on the front for nearly a half century was contrary to evidence available from 1970 that this was likely to be harmful. Systematic review of preventable risk factors for SIDS from 1970 would have led to earlier recognition of the risks of sleeping on the front and might have prevented over 10 000 infant deaths in the UK and at least 50 000 in Europe, the USA, and Australasia.

Source: Infant sleeping position and the sudden infant death syndrome.

Meta-analysis involves the statistical analysis of results from the individual studies included in a systematic review. The article What is meta-analysis? provides an overview of this research process and recognises that "the strength of conclusions from meta-analysis largely depends on the quality of the data available for synthesis. This reflects the quality of individual studies and the systematic review".  

Guidelines

Clinical guidelines are designed to support the decision-making process in patient care based on systematic reviews of clinical evidence, the main source for evidence-based care.

Clinical guidelines and trials guideFind clinical guidelines and trials in the health sciences.

Clinical trials

Clinical trials are particularly important for determining the effectiveness of interventions or therapies. The article Explaining the importance of clinical trials describes the use of randomised controlled trials.

The use of the drug thalidomide, a sleeping pill also utilised to relieve morning sickness, is an example of why it’s important to test new treatments.

James Lind’s prospective controlled trial for the treatment of scurvy in 1747 provides one of the earliest accounts of a clinical trial.

Other study types

It is not always feasible or ethical to conduct a clinical trial. Other study types can be used when ethical considerations are a factor, such as a cohort study. This was research method was first adopted by Doll and Hill in what’s known as the British Doctors Study.

Smoking had always been seen as a benign activity until researchers began to question this proposition in light of observations of smokers. In the UK, Richard Doll and Bradford Hill performed a case-controlled study in 1950 called Smoking and carcinoma of the lung: preliminary report. This was followed by the British Doctors Study, a cohort study which tracked UK doctors for 50 years.  This research proved the link between smoking and cancer.

Best evidence for different types of clinical questions

When searching for evidence, the type of clinical question helps determine the study type to look for. The table below provides a guide to finding the best evidence for your clinical question.

For all types of clinical questions, a systematic review or meta-analysis may provide the best evidence.

Examples: 

Critical appraisal is important

It’s essential to evaluate the quality of evidence. There are a number of critical appraisal tools or checklists for this, including the Critical Appraisal Skills Programme (CASP) checklists.

These Critical appraisal videos (YouTube 21m14s) examine 10 principles that can be applied when critically appraising literature.

Bias

An understanding of the role of bias in research is also fundamental in determining the quality of evidence. Smith and Noble state that:

Understanding research bias is important for several reasons: first, bias exists in all research, across research designs and is difficult to eliminate; second, bias can occur at each stage of the research process; third, bias impacts on the validity and reliability of study findings and misinterpretation of data can have important consequences for practice.

Source: Bias in research.

There are many biases that can affect health research, as mapped in the Catalogue of bias.

Health researchLearn about advanced health research techniques.

To understand and assess levels of evidence, it's helpful to have an understanding of the basic characteristics of the major evidence types, several of which are defined below. For additional evidence type definitions browse the Center for Evidence-Based Medicine Glossary

Systematic Review

The application of strategies that limit bias in the assembly, critical appraisal, and synthesis of all relevant studies on a specific topic. Systematic reviews focus on peer-reviewed publications about a specific health problem and use rigorous, standardized methods for selecting and assessing articles. A systematic review may or may not include a meta-analysis, which is a quantitative summary of the results.

Randomized Controlled Trial

An experiment in which subjects in a population are randomly allocated into groups, usually called study and control groups, to receive or not receive an experimental preventive or therapeutic procedure, maneuver, or intervention. The results are assessed by rigorous comparison of rates of disease, death, recovery, or other appropriate outcomes in the study and control groups.

Cohort Studies

Cohort studies identify a group of patients who are already taking a particular treatment or have an exposure, follow them forward over time, and then compare their outcomes with a similar group that has not been affected by the treatment or exposure being studied. Cohort studies are observational and not as reliable as randomized controlled studies since the two groups may differ in ways other than in the variable under study.

Case-Control Studies

Case-control studies are studies in which patients who already have a specific condition are compared with people who do not have the condition. The researcher looks back to identify factors or exposures that might be associated with the illness. They often rely on medical records and patient recall for data collection. These types of studies are often less reliable than randomized controlled trials and cohort studies because showing a statistical relationship does not mean that one factor necessarily caused the other.

Cross-Sectional Studies

Describe the relationship between diseases and other factors at one point in time in a defined population. Cross-sectional studies lack any information on the timing of exposure and outcome relationships and include only prevalent cases. They are often used for comparing diagnostic tests. Studies that show the efficacy of a diagnostic test are also called prospective, blind comparisons to a gold standard study. This is a controlled trial that looks at patients with varying degrees of an illness and administers both diagnostic tests — the test under investigation and the “gold standard” test — to all of the patients in the study group. The sensitivity and specificity of the new test are compared to that of the gold standard to determine potential usefulness.

Case Series and Case Reports

Case series and Case reports consist of collections of reports on the treatment of individual patients or a report on a single patient. Because they are reports of cases and use no control groups to compare outcomes, they have little statistical validity.

Definitions adapted from: https://www.cebm.ox.ac.uk/resources/ebm-tools/glossary