Thorndike (1963 - see Smith, 2003:577) views 'underachievement' as 'achievement falling below [a particular standard] what would be forecast from our most informed and accurate prediction, based on a team of predictor variables'.
According to Gorard and Smith (2004) underachievement is used to refer to achievement relative to another group, whilst Gillborn and Gripps (1996:1) suggest it is a 'relatively crude term relating to differences in group averages' and educational outcomes among different ethnic groups (Gillborn and Mirza, 2000).