Classical Test theory suggests that any assessment will only reveal an individual’s observed score, and that this is not always reflective of their ‘true’ score, as there is always something in the environment that impacts an individual’s performance (error). Assessments that are designed to determine an individual's true score are flawed as they are manmade and not infallible. Thus:
OBSERVED SCORE = TRUE SCORE + ERROR.
True Score – this is the individual’s true ability and is always constant for a particular person.
Observed Score – this is the score obtained by an individual on an assessment.
Error – anything that may have impacted an individual’s performance on a test.
By practising tests, you are minimising the error and therefore increasing your observed score. With zero error (not possible), your observed score should equal your true score.