Introduction to Psychology, 7th Edition by Clifford T. Morgan, Richard A. King, John R. Weisz, John Schopler (eBook)

13. Type 1 error

The decision to reject or retain H0 depends on the announced level of significance, α. α is a statement of risk that the researcher is willing to assume in making a decision about H0.

When H0 is true (µ0 = µtrue), 5% of all possible sample means nevertheless will lead to the conclusion that H0 is false because 5% of the sample means fall in the “rejection" region of the sampling distribution, even though these extreme means will occur (though rarely) when H0 is true. Thus, there’s a probability of .05 that H0 will be rejected when it is actually true. Rejecting a true H0 is a decision error.

Level of significance, α, gives the probability of rejecting H0 when it is actually true. Rejecting H0 when it is true is known as a Type I error.

A Type I error is getting statistically significant results “when you shouldn’t." To reduce the risk of making such an error, the researcher can set α at a lower level.

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 Sources:
Fundamentals of Statistical reasoning in Education, Theodore Coladarci, Casey D. Cobb, Edward W. Minium and Robert C. Clarke (Click for eBook)

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