Understanding the different types of evidence and application to practice

Introduction

In the first article of this series, ‘An introduction to evidence-based practice’ (Mahoney, 2025) the importance of robust clinical evidence in guiding clinical practice was emphasised. But what defines good evidence? What are the different types of evidence, and how can they be applied effectively? Identifying the most relevant and reliable evidence can be challenging, as not all evidence is of equal quality or suitable for every patient or clinical scenario (Murad et al, 2016).

McSherry et al (2006) highlighted the importance of nurses having the appropriate attitudes, skills, and knowledge to critically appraise evidence to enhance the quality of care and support informed decision-making when delivering patient care. In alignment with this, the Nursing and Midwifery (2018) Code of Conduct states that nurses must practise effectively, in line with the best available evidence. Yet despite this, interpreting evidence and applying it into clinical practice often remains problematic and challenging (Kumah et al, 2022).

Some of the challenges in implementing evidence-based practice occur at the organisational level, including a lack of resources and limited managerial support. At the individual level, barriers include insufficient understanding of evidence-based practice, time constraints, and heavy workloads (Ubbink et al, 2013; Kumah et al, 2022). There is also some confusion around the terminology related to evidence-based practice, with alternative terms such as evidence-based medicine, evidence-based nursing, and evidence-informed practice being used. Nevertheless, the key focus remains on applying the most relevant evidence to clinical practice (Ellis, 2024a).

Why is it important to be able to appraise evidence?

Rapid advances in technology, along with an evolving understanding of healthcare and disease processes, are transforming the delivery of care (Ellis, 2024a). These developments may offer more efficient and effective methods of practice. Consequently, it is no longer acceptable to continue providing care based solely on tradition or routine; the justification of ‘we have always done it this way’ is insufficient in modern clinical practice (Moule, 2021). However, it is equally important to recognise that not all emerging technologies or treatments necessarily lead to improved outcomes or represent the most cost-effective approaches to care (Ellis, 2024a). Therefore understanding, interpreting and translating evidence into clinical practice is an essential part of delivering safe, effective, and patient-centred care.

What does good evidence look like?

Good clinical evidence is credible, well-researched, and applicable to patient care. It comes from reputable sources, follows rigorous methodologies, and is peer-reviewed to ensure reliability (Lima et al, 2023).

What is the hierarchy of evidence?

The hierarchy of evidence (Figure 1) provides a rating system that helps clinicians evaluate the structure, credibility, and relevance of evidence (Murad et al, 2016). Higher levels of evidence, such as systematic reviews and randomised controlled trials, are considered more reliable because they use rigorous methodology and have a reduced risk of bias (Al Noman et al, 2024). In contrast, case studies and expert opinions rank lower on the scale (Murad et al, 2016). Nevertheless, each level of evidence can provide valuable insights, and selecting the most appropriate evidence requires careful consideration of the clinical context, including the individual’s needs and preferences, available resources, legal and ethical frameworks, and professional expertise (Ellis, 2024b). Consequently, high-level evidence, such as randomised controlled trials, may not always be the most suitable form of evidence—particularly in situations centred on human experience, such as enhancing quality of life—where qualitative or lower-tier evidence may offer more meaningful guidance (Ellis, 2024b).

Types of clinical evidence included in the hierarchy of evidence:
 

1. Systematic reviews and meta-analyses

These are considered the highest level of evidence. A systematic review gathers, analyses and summarises the findings from research papers on a particular topic (Al Noman et al, 2024). One of the most widely used systematic review databases is the Cochrane Database of Systematic Reviews which is recognised globally as producing robust reviews that summarise and interpret the best available evidence of specific healthcare interventions or treatments (Al Noman et al, 2024). A meta-analysis is a statistical technique that is often (but not always) part of a systematic review and combines numerical data from multiple studies to produce a single pooled estimate of the effect such as a treatment or intervention (Al Noman et al, 2024). Key features of meta-analysis and systematic reviews are that they:

• Summarise multiple studies to provide comprehensive insights
• Draw conclusions from included studies and usually provide more robust and accurate conclusions than a single research study.

(Al Noman et al, 2024)

2. Randomised controlled trials

A robust and well-conducted randomised controlled trial is considered the gold standard for assessing treatment efficacy (Ellis, 2024b). The experimental design of a randomised controlled trial governs how accurately that something is being tested and is the framework of how the research is conducted to ensure validity, reliability and control of bias (Schulz et al, 2010). Well-conducted randomised controlled trials reduce bias through randomisation and using controlled conditions which increases the generalisation and applicability to the wider population (Al Noman et al, 2024). The intervention design of a randomised controlled trial tells us what is being done to the participants, i.e. it refers to the treatment or action that is being tested to see if it causes an effect (Craig et al, 2008). However, inconsistencies in reporting methodologies and poor study design can lead to misinterpretation of results, especially when there is a lack of transparency regarding study conduct, allocation concealment, or blinding (Dechartres et al, 2017).
 

3. Cohort and case-control studies

Cohort studies give observational insights into risk factors and outcomes of a fixed group of individuals so there is no intervention for participants and participants are usually monitored over time (Al Noman et al, 2024). Cohort studies are useful for identifying causes and risk factors for diseases (Ellis, 2016). For instance, a cohort study could track smokers and non-smokers over time to compare the incidence of heart disease or lung cancer in each group, helping determine whether smokers are at greater risk of that condition than non-smokers.

Case-control studies retrospectively compare groups of individuals with a disease or condition to those without it, analysing their lifestyle factors and medical histories to identify potential associations with the condition (Al Noman et al, 2024). Both these types or studies are useful when randomised controlled trials are impractical or unethical (Table 1) and are relatively quick, cheap and easy to undertake (Ellis, 2016).

Table 1. Examples of potential ethical issues surrounding clinical trials (Adapted from World Medical Association, 2013; Paquette et al, 2019)
 

Harm to patients	Patients are not fully informed of risks or side effects of intervention If people get worse in the control group researchers should be able to stop the trial as early as possible if harm is emerging
Lack of informed consent	Patients should know exactly what they are consenting to Patients should be aware that they may receive the placebo instead of the treatment
Exploitation of vulnerable populations who are not able to give informed consent	For example: Children People with dementia People who are unable to understand what is being asked of them or elements of the research process Patients in critical care
Researcher bias	Researcher could potentially manipulate variables such as randomisation of patients to ensure the intervention has more favourable outcomes

4. Case reports and case series

A case report is a detailed description of a single patient’s medical history, symptoms, diagnosis, treatment, and follow-up (Al Noman et al, 2024). It is usually used to highlight something unusual or novel—like a rare disease, unexpected side effect, or a unique presentation of a common condition (Al Noman et al, 2024). A case series is a collection of case reports involving multiple patients who have similar diagnoses or treatments. Unlike more rigorous study designs, case series do not include a comparison group. This type of research can help generate hypotheses for further research (Al Noman et al, 2024).
 

5. Expert opinions and guidelines

Expert opinion in clinical practice, expert opinion refers to recommendations or judgements provided by individuals with recognised expertise in a particular medical field (Ponce et al, 2017). These opinions are often used when high-quality empirical evidence is lacking, such as in cases involving rare conditions or emerging treatments (Ponce et al, 2017).

Clinical practice guidelines are systematically developed statements designed to assist practitioner and patient decisions about appropriate healthcare for specific clinical circumstances (Kredo et al, 2016). Clinical practice guidelines are used whenever healthcare professionals need evidence-based recommendations to guide decisions about patient care. They are like high-quality roadmaps that help clinicians choose the best path for diagnosis, treatment, and management of health conditions. Clinical practice guidelines are based on population-level evidence, which might not always apply to individual patients with unique needs, preferences, or comorbidities.

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