I take the key elements in current theorising about validity to be:
Validity is based on the purpose(s) of an assessment and how effectively the interpretation and use of the results serve each purpose.
Reliability and fairness are part of validity. Unreliable results undermine confidence in their interpretation, unfairness will also lead to misinterpretation of the results. (Stobart, 2008a)
Any validity enquiry in formative assessment will place its emphasis on the first of these. The purpose is to improve earning, so effective formative assessment will do this and the ‘threats to validity’ are those things that get in the way of this. This is a consequential argument, an approach both championed (Shepard, 1997) and disputed (Popham, 1997) in test validity. The second is implicated in this – there is a need for clarity, by both teachers and learners, about what is being learned. The third, reliability, is less central as a concern in Validity in Formative Assessment AEA Europe 2008 formative assessment because any assessment of what is needed to be done will vary from learner to learner, so two learners with similar performance may receive different feedback.
Assessment is an activity that evaluates pupils’ performance.   Assessment should be employed with a reason in mind for appraising a pupil’s work. Kyriacou (DATE) states 7 purposes for assessing pupils and teachers must be skilled in shunning or restricting any possible side effect to assessment. For example, some pupils may receive feedback from assessment that is of a low level of ability and this could sap all confidence the pupil has in their learning of the subject and other areas of the school as they feel alienated from their peers who are achieving higher.