1.0     IntroductionLevels of Evidence (1)

The aim of a systematic review is to identify, evaluate and summarise the findings of all relevant individual studies, thereby making the available evidence more accessible to decision-makers and fitness professionals/practitioners.

It is important that fitness professionals understand what a systematic review is and how to interpret the various ‘versions’. Further, it is equally important that fitness professionals understand how each version’s methodology can affect the results and subsequently real world practice.

This article provides a brief overview of systematic reviews including what they are, their ‘hierarchy’ and an outline of the levels of evidence as derived from the reviews.

More detailed insight about systematic reviews can be gained by viewing the useful links and references sections at the end of the article (though don’t expect a quick read!).

2.0     What is a Systematic Review?

A systematic review is a literature review focused on a research question that tries to identify, appraise, select and synthesise all high quality research evidence relevant to that question.

Systematic reviews of high-quality randomised controlled trials are important to evidence-based medicine (CEBM, 2011). An understanding of systematic reviews and how to implement them in practice is becoming mandatory for all professionals involved in the delivery of health care. Besides health interventions, systematic reviews may concern clinical tests, public health interventions, social interventions, adverse effects, and economic evaluations (CRD, 2009; EPPI, 2010).

Systematic reviews are not limited to medicine and are quite common in other sciences where data are collected, published in the literature, and an assessment of methodological quality for a precisely defined subject would be helpful. Other fields where systematic reviews are used include psychology, nursing, dentistry, public health, occupational therapy, speech therapy, physical therapy, educational research, sociology, business management, environmental management and conservation biology.

The Centre for Evidence-based Medicine (CEBM), in my opinion, provides a well-rounded overview of systematic reviews, including the positives and negatives. CEBM also has a useful sport & exercise archive.

A fitting example of a systematic review (Stacey et al., 2010) focussed at fitness trainers can be found here: Knowledge Translation to Fitness Trainers: A Systematic Review. This study investigates approaches for translating evidence-based knowledge for use by fitness trainers. Specific questions were:

  • Where do fitness trainers get their evidence-based information?
  • What types of interventions are effective for translating evidence-based knowledge for use by fitness trainers?
  • What are the barriers and facilitators to the use of evidence-based information by fitness trainers in their practice?

2.1     Hierarchy of Study Designs to Assess the Effects of Interventions

This list is not exhaustive, but covers the main study designs. Names and definitions may differ between professions (e.g. the randomised controlled trial can be known as a randomised clinical trial).

  • Randomised Controlled Trials: the simplest form of RCT is known as the parallel group trial which randomises eligible participants to two or more groups, treats according to assignment, and compares the groups with respect to outcomes of interest. Participants are allocated to groups using both randomisation (allocation involves the play of chance) and concealment (ensures that the intervention that will be allocated cannot be known in advance). There are different types of randomised study designs, such as:
    • Randomised cross-over trials: where all participants receive all the interventions; for example in a two arm cross-over trial, one group receives intervention A before intervention B, and the other group receive intervention B before intervention A. It is the sequence of interventions that is randomised.
    • Cluster randomised trials: A cluster randomised trial is a trial where clusters of people rather than single individuals are randomised to different interventions. For example, whole clinics or geographical locations may be randomised to receive particular interventions, rather than individuals.
  • Quasi-experimental Studies: the main distinction between randomised and quasi-experimental studies is the way in which participants are allocated to the intervention and control groups; quasi-experimental studies do not use random assignment to create the comparison groups.
    • Non-randomised controlled studies: Individuals are allocated to a concurrent comparison group, using methods other than randomisation. The lack of concealed randomised allocation increases the risk of selection bias.
    • Before-and-after study: comparison of outcomes in study participants before and after the introduction of an intervention. The before-and-after comparisons may be in the same sample of participants or in different samples.
    • Interrupted time series: interrupted time series designs are multiple observations over time that is ‘interrupted’, usually by an intervention or treatment.
  • Observational Studies: a study in which natural variation in interventions or exposure among participants (i.e. not allocated by an investigator) is investigated to explore the effect of the interventions or exposure on health outcomes.
    • Cohort study: a defined group of participants is followed over time and comparison is made between those who did and did not receive an intervention.
    • Case-control study: groups from the same population with (cases) and without (controls) a specific outcome of interest, are compared to evaluate the association between exposure to an intervention and the outcome.
    • Case series: description of a number of cases of an intervention and the outcome (without comparison with a control group). These are not comparative studies.
  • Individual Patient Data (IPD) Meta-analysis: is a specific method of systematic review. Instead of extracting data from study publications, the original research data for each participant is obtained directly from the researchers responsible for each included study. These data can then be collated and re-analysed centrally and, if appropriate, combined in meta-analyses. The approach has been used extensively in cancer and cardiovascular disease and is becoming used more frequently in many other areas of health.
  • Prospective Meta-analysis: involves selecting a group of studies for inclusion in a meta-analysis before the results of those studies are known. Because decisions about relevant outcomes and subgroups are made in advance, there is no opportunity for selecting studies on the basis of their findings, thereby preventing publication and selection biases. It may also mean that investigators can agree on consistent study methods and data structures that will facilitate the subsequent meta-analysis. Analyses are generally done using IPD. However, as with IPD meta-analysis, prospective meta-analyses may require the close collaboration of several independent research groups, which can present various logistical challenges.
  • Reviews of Reviews: this describes a systematic review that includes only other systematic reviews. In theory the systematic reviews included in the review should have covered most of the primary studies available. Reviews of reviews are likely to be helpful when a review question is very broad and a number of systematic reviews have already been conducted in the topic area. However, the different inclusion criteria adopted by the various reviews can make synthesis and interpretation problematic.
  • Scoping Reviews: a scoping review determines the size and nature of the evidence base for a particular topic area, which can in turn be used to identify gaps in the literature and make recommendations for future primary research. The literature search should be as extensive as possible, including a range of relevant databases, hand-searching and attempts to identify unpublished literature. Scoping reviews differ from standard systematic reviews in that they do not attempt to synthesise the evidence. A scoping review might be useful to research bodies that are planning a primary study, or to assess the feasibility of a full systematic review. It is not appropriate to use a scoping review to answer a clinical question.
  • Rapid Evidence Assessments: are used to summarise the available research evidence within the constraints of a given timetable, typically three months or lessLevels of Evidence. Rapid evidence assessments differ from full systematic reviews in terms of the time constraints and consequently there are limitations on the extent of the literature searches and other review activities. Whilst attempting to be as comprehensive as possible, rapid evidence assessments usually make compromises to meet their tight deadlines; therefore they may fail to identify potentially relevant studies. They are useful to policy-makers who need to make decisions quickly, but should be viewed as provisional appraisals, rather than full systematic reviews.

3.0     Five Levels of Evidence

The general consensus regarding the hierarchy for the five levels of evidence is outlined in Table 1. Level 1 is considered the best form of evidence in contrast to level 5 which is viewed as the least best option.

Table 1: Description of the five levels of evidence


Research Design




Randomised Controlled Trial (RCT) More than 1 Higher RCT: includes within subjects comparison with randomised conditions and cross-over designs.


RCT 1 Higher RCT


Prospective controlled trial Prospective controlled trial (not randomised).
Cohort Prospective longitudinal study using at least 2 similar groups with one exposed to a particular condition.


Case control A retrospective study comparing conditions, including historical controls.


Pre-post A prospective trial with a baseline measure, intervention, and a post-test using a single group of subjects.
Post-test A prospective post-test with two or more groups (intervention followed by post-test and no re-test or baseline measurement) using a single group of subjects.
Case Series A retrospective study usually collecting variables from a chart review.


Observational Study using cross-sectional analysis to interpret relations. Results from observational studies may not adequately account for confounding and cannot be used to infer causality.
Clinical Consensus Expert opinion without explicit critical appraisal, or based on physiology, biomechanics or ‘first principles’.
Case Report Pre-post or case series involving one subject

3.1     Design Diversity

Design diversity is the combining of evidence from multiple research designs.

4.0     Useful Links

  • What is a Systematic Review? Authored by Pippa Hemingway & Nic Brereton (Medical Sciences Division, University of Oxford) and published by Hayward Group Ltd in 2009.
  • Systematic Reviews. Produced and published by King’s College London Library Services in May 2014.

5.0     References

EPPI (Evidence for Policy and Practice Information and Co-ordinating Centre) (2010) EPPI-Centre Methods for Conducting Systematic Reviews. Available from World Wide Web: http://eppi.ioe.ac.uk/cms/LinkClick.aspx?fileticket=hQBu8y4uVwI%3D&tabid=88. [Accessed: 12 April, 2012].

CEBM (Centre for Evidence-based Medicine) (2014) OCEBM Levels of Evidence. Available from World Wide Web:  http://cebm.minervation.net/ocebm-levels-of-evidence/. [Accessed: 14 July, 2014].

CRD (Centre for Reviews and Dissemination) (2009) Systematic Reviews: CRD’s Guidance for Undertaking Reviews in Health Care. 3rd ed. Available from World Wide Web: http://www.york.ac.uk/inst/crd/pdf/Systematic_Reviews.pdf. [Accessed: 14 July, 2014].

Stacey, D., Hopkins, M., Adamo, K.B., Shorr, R. & Prud’homme, D. (2010) Knowledge Translation to Fitness Trainers: A Systematic Review. Implementation Science. 5(28). doi:10.1186/1748-5908-5-28.

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