UT Knoxville STAT 201 - Chapter 21 (29 pages)

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Chapter 21 Chapter21 Presentation 1213 More About Tests Copyright 2009 Pearson Education Inc 1 Zero In on the Null To perform a hypothesis test the null hypothesis must be a statement about the value of a population parameter We then use this hypothesized value to help us compute the probability that the observed sample statistic or something even farther from the hypothesized value will occur Chapter21 Presentation 1213 Copyright 2009 Pearson Education Inc 2 Alternative Hypothesis How do we choose the alternative hypothesis From what we see in the data From the research question Chapter21 Presentation 1213 Copyright 2009 Pearson Education Inc 3 How to Think About P Values A P value is a conditional probability the probability of the sample results or results more unusual than that given that the null hypothesis is true Be careful to interpret the P value correctly Chapter21 Presentation 1213 Copyright 2009 Pearson Education Inc 4 What to Do with a High P Value When we see a small P value we could continue to believe the null hypothesis and conclude that we just witnessed a rare event Instead we trust the data and use it as evidence to reject the null hypothesis However big P values just mean what we observed isn t surprising assuming the null hypothesis is true so we have no reason to reject the null hypothesis Chapter21 Presentation 1213 Copyright 2009 Pearson Education Inc 5 Alpha Levels So far we have defined a small p value as one that is 0 05 or less and a big p value is one that is above 0 05 Is this always the cut off between small and big What should one consider before making the cut off between small and big something different Chapter21 Presentation 1213 Copyright 2009 Pearson Education Inc 6 Alpha Levels cont Although a common cut off we don t have to define rare event as 0 05 probability or less we can pick any threshold we like If our P value falls below our threshold we ll reject the null hypothesis We call such results statistically significant The