Step three - assessing the quality

Step 3 is about assessing the quality of the included studies. Quality refers to the internal validity. The internal validity of a study refers to the degree which its results are likely to be free from bias.

Bias

'Bias is any process at any stage of inference tending to produce results that differ systematically from the true values'.

There are four main sources of systematic bias in trials of the effects of healthcare.

• Selection bias - resulting from the way comparison groups are assembled
• Performance bias - systematic differences in care given to the participants
• Detection bias - systematic differences between the comparison groups in outcome assessment
• Attrition bias - systematic differences between the comparison groups in the loss of participants in the study