Final answer:
For a test to be perfectly valid, its validity coefficient should be +1. In practice, achieving a high validity coefficient such as 0.71, which corresponds to a coefficient of determination of at least 0.5, is considered strong validity.
Step-by-step explanation:
If a test is perfectly valid, the measure of its validity is expressed through a statistic known as the validity coefficient. A validity coefficient ranges from -1 to +1. For a test to be perfectly valid, indicating that it measures exactly what it is intended to measure without error, its validity coefficient would need to have a value of +1.
However, it's important to understand that a high validity coefficient is often more realistic due to the complexities and variables involved in testing scenarios. In the context of evaluating the significance of the validity coefficient, researchers use statistical tests to determine if the observed correlation coefficient significantly differs from zero. If it doesn't, then the correlation is deemed not significant.
Moreover, a validity coefficient of +1 is rare, and in practice, a coefficient of 0.71 or higher is often viewed as indicative of strong validity since it corresponds to a coefficient of determination of at least 0.5, explaining at least 50% of the variance in scores.