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DOH321 Research Skills Guide: Critical Appraisal & CASP

What is Critical Appraisal?

""In the Evidence-Based Practice process, and especially in the process of evaluating primary research, we need to critically appraise certain aspects of the evidence we have located. Even when we have found a study that is the right sort of study for our clinical question, and is of the appropriate level in the hierarchy, we still need to question its relevance, validity, and conclusions. The three main questions to be asked are:

  • Is the study relevant to our clinical question?
  • How well was the study done?
  • What do the results mean and are they statistically valid (and not just due to chance)?

Introduction to Critical Appraisal


The best way to gauge the relevance of a primary study is to compare the PICO. Remember that PICO stands for:

  • Patient / Population / Problem
  • Intervention
  • Comparison
  • Outcome.

You will rarely get an exact match, so you need to decide if the PICO matches closely enough to assist with your clinical decision.

Meaning and Value

Questions to ask here include:

  • Were statistical tests applied and did they result in a significant impact from the intervention?
  • Could the result have been due to chance, or to something else in the study which was not controlled?
  • What does the study show? What does it mean and what is its value?
  • What do the results mean for a particular context in which a decision is being made?

Trustworthiness - Bias and Confounding Factors

The quality of primary research often depends on the extent to which the methodology prevented the result from being affected by bias and confounding factors.


"Bias is the degree to which the result is skewed away from the truth" . Bias can occur in many ways, and can occur in the selection of subjects for study, allocation to groups, and measurement of results, but the best way to prevent it is to keep details of interventions hidden from participants and also researchers (Glasziou, Salisbury, & Del Mar, 2007, p. 75). This is called blinding.

Confounding factors

"Confounding factors are patient features and other possible causal factors, apart from the one that is being measured, that can affect the outcome of the study." These can be eliminated or reduced by ensuring that groups are matched as closely as possible at the start of the study, and managed in the same way in the course of the study (Glasziou et al., 2007, p. 76).

One way to quickly appraise a study's trustworthiness is to use the RAMMbo mnemonic. This looks at the elements of Recruitment, Allocation, Maintenance, and Measurement (blinding and objectivity):

R Recruitment Were the subjects in the study representative of the target population? Were there enough subjects to make the study valid?
A Allocation Were the subjects randomly (and "blindly") allocated to groups?
M Maintenance Was the status of the study group and the control group maintained throughout the trial? Were they treated the same way apart from the intervention?
Mbo Measurement (blinding, objective measures) Were the subjects (and researchers) blinded to the intervention and were the outcomes measured objectively?  Was bias eliminated as much as possible?

Source: Glasziou, P., Del Mar, C., & Salisbury, J. (2009). Evidence-based practice workbook: Bridging the gap between health care research and practice (2nd ed.). Retrieved from ProQuest Ebook Central

Further Reading: Critical Appraisal Tools

  • The Critical Appraisal Skills Programme (CASP) is part of Better Value Healthcare, a training organisation led by Professor Sir Muir Gray, and based in Oxford. It is best known for its checklists, a set of eight critical appraisal tools designed to be used when reading research.