A. Validity is the extent which an instrument measures what it would measure. Validity can be in different forms such as content, face, criterion-related validity, construct, and convergent validity. Reliability is the extent which a test, observation or measurement procedure uses to produce the same results on repeated trials. It is considered the stability or consistency of scores over time. Both validity and reliability are critical to measurement because reliability is a necessary but not sufficient condition for validity, but in order to obtain validity, you must have reliability. A measure can be reliable but not valid. For instance, some questions when asked generate reliable answers, but those answers cannot be measured. “What is your favorite game?” The answer might be constant and reliable but not measurable. You can prove validity to say it is correct. It is better to validate an issue which is being measured properly rather than rely on it by making assumptions.
B. Content validity is the most difficult type of validity to determine because the data collection instrument must adequately cover the topic through investigative questioning to be valid. Also, determination of validity is based on judgment.
C. A valid measurement is reliable, but a reliable measurement may not be valid because the response to a question such as “What is your favorite movie” might remain stable over time, demonstrating a highly reliable score, but the question might not be valid while attempting to measure job satisfaction. Reliability test yields consistent scores, but reliability does not guarantee validity.
D. Stability and equivalence are the same. According to Cooper and Schindler, “While stability is concerned with personal and situational fluctuations from one time to another, equivalence is concerned with variations at one point in time among observers and samples of items” (Cooper & Schindler, 2011). In stability if an observation procedure is stable it might give the same reading on a particular person when repeated one or more times, and in equivalence, if a person is classified the same way by each test, then the tests are good equivalence (Cooper & Schindler, 2011). There is a similarity to both stability and equivalence.