The estimate of the ‘error’ associated with pupils’ obtained score when compared with their hypothetical ‘true’ score. The SEM, which varies from test to test, should be given in the test manual. The band of scores in which we can be fairly certain the ‘true’ score lies, can be calculated from these figures. For example, we can be 95 per cent certain that a pupils’ true score lies in the range ‘obtained score plus or minus 2 SEM’ and 99 per cent certain that it lies in the range ‘obtained score plus or minus 3 SEM’.