This is a term (in the US more commonly referred to as Backwash) used in testing and assessment to describe the effect on the classroom of tests that the learners will take. Washback may affect e.g. the syllabus, methodology, interaction patterns, attitudes to learning etc., and can be positive or negative.
In some countries education authorities deliberately introduce new elements into tests so that they will be used in the classroom. In other words they are relying on the washback effect of a test to bring about change in the classroom. Examples of this might be the introduction of speaking tests or the use of tasks in speaking tests. Washback is sometimes known as ‘Backwash’ and is contrasted with ‘Impact’.
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Taylor, L. (2005) Key Concepts in ELT: Washback and Impact. ELT Journal 59/2.