Which measure would be used for testing the consistency of an instrument at two different times?

Test-retest reliability is the measure used to assess the consistency of an instrument over time. This reliability testing involves administering the same assessment to the same group of individuals on two separate occasions and then determining the degree to which the scores are similar. A high correlation between the two sets of scores indicates that the instrument yields stable results over time, which is crucial for ensuring that any changes in scores do indeed reflect true changes rather than fluctuations in measurement.

This form of reliability is particularly important in longitudinal studies or any research where the stability of a measurement is vital. By establishing test-retest reliability, researchers can be more confident in the dependability of their tools, ensuring that they can accurately track changes or compare results across different time points.

The other measures mentioned, while important, do not specifically address the concept of consistency over time. For instance, split-half reliability assesses the internal consistency of an instrument by comparing results from two halves of the same test, whereas Cronbach's alpha measures internal consistency across all items in a single test. Parallel reliability involves comparing two different instruments measuring the same construct. Each of these concepts targets specific aspects of reliability, but test-retest reliability specifically focuses on consistency across time intervals, making it the appropriate choice for this scenario.