Final answer:
A researcher fails to reject a false null hypothesis is known as a Type II error.
Step-by-step explanation:
A researcher fails to reject a false null hypothesis is known as a Type II error. This occurs when the researcher incorrectly concludes that there is no significant difference or effect, when in reality there is. It is important to note that a Type II error is different from a Type I error, which occurs when the researcher incorrectly rejects a true null hypothesis.