Omar Bennett
Evidence is information that is systematically collected through established research methods. It can be qualitative or quantitative and come from a range of sources, such as academic journal articles, published literature, analysis of statistics and data, and reports from government and non-government organisations. Evidence-based research is the use of prior research in a systematic and transparent way to inform a new study. It involves understanding what evidence is available and what types of evidence are available. Evidence hierarchies have been developed to help researchers quickly find the best evidence, with the most useful hierarchies developed by the Oxford Centre for Evidence-based Medicine. Evidence types include systematic reviews, randomised controlled trials, quasi-experimental studies, case studies, and qualitative studies.
| Characteristics | Values |
|---|---|
| Research design | Quantitative, qualitative, mixed-method |
| Sources | Academic journal articles, published literature, analysis of statistics and data, reports from government and non-government organisations |
| Data type | Numerical, non-numerical |
| Rigour | Strength, precision, independence |
| Hierarchy of evidence | Systematic reviews, randomised controlled trials, quasi-experimental studies, case studies, qualitative studies |
| Meta-analysis | Synthesis of multiple independent studies and reviews |
| Systematic reviews | Summarise evidence to support clinical decisions, undertaken by a team of experts, regularly updated |
| Randomised controlled trials | Compare treatment and control groups, greater control over factors influencing outcomes |
| Quasi-experimental studies | Similar to RCTs but without random allocation to groups |
| Case studies | Describe events or issues in everyday contexts, capture information on 'how', 'what' and 'why' questions |
| Qualitative studies | Provide in-depth information about people's experiences, behaviours, beliefs or attitudes |
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What You'll Learn
Hierarchy of evidence
Evidence is a cornerstone of evidence-based medicine (EBM) and research. The hierarchy of evidence is a core principle of EBM, ranking study types based on the strength, rigour, and precision of their research methods. The higher up the hierarchy a study is, the more rigorous its methodology is, and the more likely it is to minimise bias.
Systematic reviews, with or without meta-analyses, are generally considered to provide the best evidence for all question types. They are based on the comprehensive findings of multiple studies and are undertaken by a team of experts. However, the position of systematic reviews at the top of the hierarchy is not absolute. For example, a rigorous systematic review can take years to complete and may be superseded by more recent evidence. A large, well-conducted randomised controlled trial (RCT) may provide more convincing evidence than a systematic review of smaller RCTs.
If a current, well-designed systematic review is unavailable, researchers move to primary studies to answer their question. The best research design depends on the question type. For example, if treatment effects found in well-designed cohort studies are sufficiently large and consistent, those cohort studies may provide more convincing evidence than the findings of a weaker RCT.
Different hierarchies exist for different question types, and even experts may disagree on the exact rank of information in the evidence hierarchies. For example, the Oxford Centre for Evidence-based Medicine (OCEBM) has developed a hierarchy of evidence designed to be used as a shortcut for busy clinicians, researchers, or patients to find the likely best evidence. The levels of evidence were originally described in a report by the Canadian Task Force on the Periodic Health Examination in 1979, which was further expanded upon in 1989.
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Systematic reviews
The data extracted for a systematic review can encompass various elements, such as the methodology employed, participant demographics, funding sources, and outcomes. This information is then synthesised using complex statistical techniques to derive an overall result. Visual representations, like blobbograms or forest plots, can help illustrate the data and its implications.
There are different types of systematic reviews, including quantitative, qualitative, and mixed-methods reviews. While quantitative reviews often involve a meta-analysis of available data, qualitative reviews focus on synthesising textual data. Mixed-methods reviews combine both quantitative and qualitative data, adhering to stringent standards for evidence gathering and analysis.
Additionally, scoping reviews are a relatively new approach that differs from systematic reviews. Scoping reviews aim to identify knowledge gaps, scope the literature, clarify concepts, and investigate research conduct. They can be useful precursors to systematic reviews, helping to refine the research question and determine the feasibility of a more comprehensive review.
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Meta-analysis
Meta-analyses are commonly used in some fields of research, such as medicine, and are rarer in others. They are often used in cancer studies, for example, as they can be based on multiple clinical trials. However, they can also be useful in other fields where clinical trials would be impractical or unethical.
Meta-analyses can be time-consuming and challenging to conduct. Locating all studies is one of the most difficult aspects of a meta-analysis, and a structured plan is necessary to manage the large number of papers. Researchers must decide whether to include studies from grey literature, which is research that has not been formally published, such as conference abstracts and dissertations. This can help reduce publication bias, but the methodological quality of grey literature is often lower.
When conducting a meta-analysis, it is important to include published and unpublished results to avoid publication bias. Data collection is another critical step, and appropriate keywords and search limits must be identified. A data collection form can be used to standardise the process, and it is important to collect information on important study characteristics, such as the mean age of participants.
Overall, meta-analyses are a powerful tool for synthesising data from multiple studies and can be used to inform research grant proposals, treatment guidelines, and health policies.
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Research design
Evidence-based research is the use of prior research in a systematic and transparent way to inform a new study. It is important to understand the different types of evidence available and how they can be used to design a new study. The research design chosen will depend on the research question and the type of evidence available.
The hierarchy of evidence is a core principle of Evidence-Based Practice (EBP) and is used to locate the best evidence. The hierarchy ranks study types based on the rigour of their research methods, with the most rigorous methodologies positioned higher in the hierarchy. Well-conducted systematic reviews are generally considered to provide the best evidence for all question types as they are based on the findings of multiple studies. However, the position of systematic reviews at the top of the hierarchy is not absolute, as more recent evidence may supersede the findings of a systematic review.
Systematic reviews use a reproducible method to search for, critically appraise, and synthesise the findings of multiple studies on a specific topic. They are typically undertaken by a team of experts, including content experts, experienced searchers, bio-statisticians, and methodologists. Systematic reviews can contain a meta-analysis, which is a particular type of systematic review that focuses on selecting and reviewing quantitative research. Meta-analyses combine the results of several independent studies and reviews to produce a synthesis, providing some control between studies and generalised application to the population.
Randomised Controlled Trials (RCTs) are another type of study that is often considered to provide strong evidence. RCTs compare a group receiving a service or treatment to a control group that does not to see if there is a significant difference in their outcomes. RCTs randomly assign participants to the treatment and control groups, allowing for greater control over factors that might influence outcomes. However, a large and well-conducted RCT may provide more convincing evidence than a systematic review of smaller RCTs.
Other types of studies that can be used to collect evidence include case studies, qualitative studies, and quasi-experimental studies. Case studies are used to explain, describe, or explore events or issues in their everyday contexts and can provide insight into 'how', 'what', and 'why' questions. Qualitative studies collect and analyse non-numerical data, providing in-depth information about people's opinions, experiences, behaviours, beliefs, and attitudes. Quasi-experimental studies are similar to RCTs but do not randomly allocate participants to treatment and control groups.
The choice of research design depends on the research question and the availability of evidence. It is important to consider the rigour of the research methods and the potential for bias when selecting a research design. By understanding the hierarchy of evidence and the different types of studies available, researchers can design studies that effectively utilise evidence-based research approaches.
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Evidence-based research approach
Evidence-based research is a systematic approach to conducting new studies that build upon prior research. It involves justifying the research question, designing the study, and placing the results in the context of earlier studies. This approach ensures that research is valid, efficient, and accessible, answering questions that are relevant and meaningful.
The hierarchy of evidence is a fundamental principle in evidence-based research. It provides a top-down framework for locating the best available evidence, with different levels of evidence based on research design, quality, and applicability. Well-conducted systematic reviews, which synthesise multiple studies, are generally considered the highest form of evidence as they provide a comprehensive overview of current evidence. However, the position of systematic reviews in the hierarchy is not absolute, as more recent evidence or a well-designed randomised controlled trial (RCT) may provide more convincing evidence.
Systematic reviews involve a rigorous process undertaken by a team of experts, including content experts, searchers, biostatisticians, and methodologists. They aim to summarise and analyse large amounts of data and information from multiple studies, addressing a specific clinical question. Meta-analyses, a type of systematic review, focus on quantitatively synthesising research findings to enable generalised application to populations.
When conducting evidence-based research, it is important to consider the different types of evidence available, including quantitative, qualitative, and mixed-method research. Quantitative research deals with numerical data and is useful for testing causal relationships and making predictions. Qualitative research, on the other hand, collects and analyses non-numerical data, providing valuable insights into people's opinions, experiences, behaviours, and attitudes. Mixed-method research combines both quantitative and qualitative approaches.
By adopting an evidence-based research approach, researchers can avoid unnecessary clinical research, which poses risks to patients, limits funding for relevant studies, and may diminish trust in research. This approach ensures that new studies are informed by prior research, justifying the research question and contributing meaningful insights to the existing body of knowledge.
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Frequently asked questions
The best available evidence is determined by the hierarchy of evidence, a core principle of Evidence-Based Practice (EBP). The hierarchy of evidence ranks study types based on the rigour of their research methods. For example, well-done systematic reviews are generally considered to provide the best evidence for all question types as they are based on the findings of multiple studies.
Evidence can be quantitative, qualitative, or mixed-method. Quantitative research collects and analyses numerical data, while qualitative research collects and analyses non-numerical data. Mixed-method research encompasses both qualitative and quantitative data collection and analysis. Other types of evidence include case studies, meta-analyses, and practice guidelines.
Evidence-based research is the use of prior research in a systematic and transparent way to inform a new study. It helps to answer questions that matter in a valid, efficient, and accessible manner. It also ensures that research is ethical and relevant, preventing unnecessary clinical research that could put patients at risk.
