Blinding is where information about the assignment of participants to their experimental group (e.g. control or treatment) is concealed from the evaluator, the participants, or other people involved in the study until it is complete.
Blinding can be introduced at various points in an evaluation. In EEF-funded evaluations the following are blinded:
Failure to blind can introduce bias. For example, an exam marker may behave differently if they know that their examinees are receiving an intervention. If they do not like the intervention, they may subconsciously mark the intervention group lower than the control group. Even if a marker does their best to remain fair and objective, their own preconceptions of an intervention can still affect their marking and introduce bias, without them realising.