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Sunday, 10th December 2017
Types of evidence

Evidence arises from undertaking research, often defined as the search for new knowledge. The research process takes three main forms:

  • Exploratory research, which structures and identifies new problems;
  • Constructive research, which develops solutions to a problem, and;
  • Empirical research, which tests the feasibility of a solution using empirical evidence.

Research can also fall into two distinct types:

  • Primary research (collection of data that does not exist yet ), and;
  • Secondary research (the summarising, collation or synthesis of existing research).

Depending upon the nature of the question being asked, research can be conducted using qualitative or quantitative research methods. Qualitative research methods are very useful where a research question lends itself to exploring a processes and / or meanings in rich detail, the 'whys' rather than the 'how manys'.

Quantitative research methods, on the other hand, can be classified into experimental, quasi-experimental and observational, with the latter often subdivided into descriptive and analytic studies. In instances where little is known about the occurrence of a problem, descriptive studies are more suitable; where the need is to examine associations between dependent and independent variables, analytic studies are most suitable.

In an era of evidence-based medicine, the different types of evidence of efficacy (i.e. did it work) available are graded according to the strength of their freedom from various biases in medical research that can weaken research findings. There are a number of grading schemes in existence. One from the United Kingdom is shown below:

Evidence of the efficacy of an intervention

Level of evidence

Type of evidence

1++

High quality meta-analyses, systematic reviews of randomised controlled trials (RCTs) (including cluster RCTs), or RCTs with a very low risk of bias.

1+

Well conducted meta-analyses, systematic reviews of RCTs, or RCTs with a low risk of bias.

1-*

Meta-analyses, systematic reviews of RCTs, or RCTs with a high risk of bias.

2++

High quality systematic reviews of, or individual high quality non-randomised intervention studies (controlled non-randomised trial, controlled before-and-after, interrupted time series), comparative cohort and correlation studies with a very low risk of confounding, bias or chance.

2+

Well conducted, non-randomised intervention studies (controlled non-randomised trial, controlled before-and-after, interrupted time series), comparative cohort and correlation studies with a low risk of confounding, bias or chance.

2-*

Non-randomised intervention studies (controlled non-randomised trial, controlled before-and-after, interrupted time series), comparative cohort and correlation studies with a high risk of confounding, bias or chance.

3

Non-analytical studies (e.g. case reports, case series).

4

Expert opinion, formal consensus.

*Studies with a level of evidence (-) should not be used as basis for making recommendations.

Source : adapted from SIGN (2001) Scottish Intercollegiate Guidelines Network: Guidelines methodology . www.sign.ac.uk/methodology .