11.1 Types of Evidence

Published works in journals are generally divided into three categories: original research, secondary research, and opinion or commentary.

11.1.1 Original Research

Original research – also called primary research – directly reports on the findings from a study. This work is generally considered to be novel, and even with peer review is subject to error that requires close scrutiny.

The quality of this evidence is governed by a number of factors, which include:

• Study type (experimental, observational)
• Study design (blinding, use of controls etc)
• Systematic bias (sampling distribution, sampling size, etc)
• Other bias (financial conflicts, cognitive biases, etc)
• Level of reporting (access to protocols or registrations, data availabability, code availability, etc)
• Appropriate choice of statistical analysis

11.1.2 Secondary Research

Secondary research – also called synthesis research – cumulatively reports on the findings of original research. This kind of reporting attempts to establish the extent of knowledge on a specific topic (systematic review), identify if sufficient evidence exists on a specific topic to suggest the evidence is conclusive (meta analysis), scope the degree or level of evidence available in a specific field of inquiry (scoping review), or offer high level commentray backed by some evidence on a specific topic (narrative review).

The quality of this evidence is governed by a number of factors, which include:

• Study type (meta-analysis, systematic review, scoping review, narrative review)
• Systematic bias (reasonable attempts to find all published literature, attempts to find unpublished results, etc)
• Other bias (financial conflicts, cognitive biases, etc)
• Level of reporting (access to protocols or registrations, data availabability, code availability, etc)
• Appropriate choice of statistical analysis (for meta analysis)

11.1.3 Opinion & Commentary

Commentaries offer ‘expert’ opinion on topics, sometimes suggesting conclusions, sometime suggesting future directions. While an important part of science communication, these neither report comprehensively on a given study nor systematically evaluate existing published research. These should not be treatee as sources of evidence.