Sources of error associated with time-sampling (e.G., practice effects, carry-over effects) are best expressed in test-retest reliability coefficients, whereas error associated with the use of particular items is best expressed in internal consistency reliability coefficients.
Test-Retest Reliability measures test consistency — the reliability of a test measured over time. In other words, give the same test twice to the same people at different times to see if the scores are the same. For example, test on a Tuesday, then again the following Tuesday. The two scores are then correlated.
Internal consistency reliability defines the consistency of the results delivered in a test, ensuring that the various items measuring the different constructs deliver consistent scores. For example, a French test is divided into vocabulary, spelling, punctuation and grammar. The internal consistency reliability test provides a measure that each of these particular aptitudes is measured correctly and reliably.
