Abstract

Classical Test Theory deals with the effect of both unsystematic and systematic influences on the observed test result.
The theory assumes that each test score contains a True component (T) and an Error component (E). When measuring a psychological construct, unsystematic errors occur. These unsystematic errors can be the result of a range of factors, for instance distractions during testing, physical wellbeing of the candidate at the time of testing, goo fortune or bad luck. Sometimes these influences have a positive effect on the test result; other times they have a negative effect. In other words, they cause a band (range) of error around the True score. The True score can be seen as the systematic component of the raw score obtained. Classical Test Theory assumes that the errors in measurement are evenly dispersed (spread) around the average.
Based on Classical Test Theory, concepts related to reliability (how accurately and consistently a test measures a construct) and validity (does it measure what it is supposed to measure) are introduced, as well as how they relate to each other. Finally, the concept of standard scores and how to interpret test results from a Classical Test Theory perspective are discussed.