Which of the following tests is NOT used to estimate the stability of an instrument?

Face validity is a type of validity that assesses whether a test appears to measure what it is supposed to measure, based on subjective judgment. It is primarily concerned with the appearance of relevance and appropriateness of the instrument's items but does not evaluate the consistency or stability of the measurements over time.

On the other hand, test-retest reliability, parallel reliability, and internal consistency all serve to estimate the stability and reliability of an instrument. Test-retest reliability assesses the consistency of results when the same test is administered at different times. Parallel reliability compares the results of two different versions of a test that measure the same construct. Internal consistency evaluates the correlation between different items within the same test, indicating whether they measure the same underlying concept consistently.

Thus, while the other options contribute to understanding the reliability and stability of an instrument's results, face validity does not pertain to stability in the same manner, making it the option that is not used to estimate the instrument's stability.