The researcher has made a type II error. It's because the researcher has made a decision that a certain pill is not effective at treating depression when, in reality it is.
What is a type II error?
A type II error is a statistical term known as an error of omission. This statistical term is used within the context of hypothesis testing that describes the error that occurs when someone fails to reject a null hypothesis that is actually false. The type II error is mainly caused by the statistical power of a test being low and will occur if the statistical test is not powerful enough.
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