What is a null hypothesis?

• A. A statement that there is always an effect or difference.
• B. A theory generating statement.
• C. An experimental condition.
• D. A statement that there is no effect or difference; tested in hypothesis testing.

Answer: (D)

The idea being tested is whether there is an effect or difference, which is what the null hypothesis states. It is a statement of no effect or no difference about a population, and it provides the baseline that hypothesis testing uses to evaluate the data. When you perform a test, you assess whether the observed results are plausible under this no-effect assumption. If the data lead to a very small p-value, you have evidence against the null and support the idea that there is some effect or difference. If the p-value isn’t small enough, you do not reject the null, meaning the data don’t provide strong evidence of an effect, though that doesn’t prove there isn’t one.

The other descriptions don’t fit as well. Saying there is always an effect contradicts the idea of testing a no-effect claim. A theory-generating statement describes a broader hypothesis or theory, not the specific baseline claim tested in a formal hypothesis test. An experimental condition refers to the manipulated setup itself, not the statistical claim about the population.