Box 8.11.a: A note on blinding in clinical trials

In general, blinding (sometimes called masking) refers to the process by which study participants, health providers and investigators, including people assessing outcomes, are kept unaware of intervention allocations after inclusion of participants into the study. Blinding may reduce the risk that knowledge of which intervention was received, rather than the intervention itself, affects outcomes and assessments of outcomes.


Different types of people can be blinded in a clinical trial (Gøtzsche 1996, Haahr 2006):

  1. participants (e.g. patients or healthy people);

  2. healthcare providers (e.g. the doctors or nurses responsible for care);

  3. outcome assessors, including primary data collectors (e.g. interview staff responsible for measurement or collection of outcome data) and any secondary assessors (e.g. external outcome adjudication committees);

  4. data analysts (e.g. statisticians); and

  5. manuscript writers.


The first two type of people are addressed in the tool by the item ‘Blinding of participants and personnel’. The third is addressed by the item ‘Blinding of outcome assessment’. The last two are not explicitly covered by the tool.