Final answer:
The size of the validity coefficient necessary for a test to be considered valid depends on the test's type and purpose. There is no specific numerical threshold, and context, such as the type of validity and the testing stakes, is essential for determining adequacy.
Step-by-step explanation:
The question at hand asks how big a validity coefficient must be for a test to be considered valid. The answer to this question is that it depends on the type of test and its purpose (option b). There is no universally accepted numeric threshold that defines a valid test; rather, the requisite size of the validity coefficient will vary depending on the specific context in which the test is used. In making this determination, researchers might consider the type of validity they're assessing—whether it's content validity, predictive validity, construct validity, etc.—as well as the stakes of the testing situation.
For instance, a high-stakes employment screening test might require a higher validity coefficient than a low-stakes personality assessment. What is important is that the test is considered sufficiently valid if it accurately measures what it's intended to measure, taking into account the correlation coefficient between the test scores and the criterion it is supposed to predict. While a correlation of 0.71 or higher is needed to have a coefficient of determination of at least 0.5, this could be considered a strong relationship, and in some cases, even a lower coefficient might be acceptable if the test serves its purpose effectively.