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# Week 5

Week 5. POWER AND SAMPLE SIZE. Terminology : Normal Samples. Power is the probability of rejecting a false null hypothesis. Power should be close to one. N1 and N2 are the number of items sampled from each population. To conserve resources, they should be small.

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## Week 5

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1. Week 5 POWER AND SAMPLE SIZE

2. Terminology: Normal Samples • Power is the probability of rejecting a false null hypothesis. Power should be close to one. • N1 and N2 are the number of items sampled from each population. To conserve resources, they should be small. • Alpha is the probability of rejecting a true null hypothesis. It should be small. • Beta is the probability of accepting a false null hypothesis. It should be small. • Mean1 is the mean of populations 1 and 2 under the null hypothesis of equality. • Mean2 is the mean of population 2 under the alternative hypothesis. The mean of population 1 is unchanged. • S1 and S2 are the population standard deviations. They represent the variability in the populations.

3. Terminology: Counts/Proportions • Power is the probability of rejecting a false null hypothesis. It should be close to one. • Ni is the size of the sample drawn from the ith population. • Allocation Ratio is N2/N1 so that N2 = N1 x R. • Alpha is the probability of rejecting a true null hypothesis. It should be small. • Beta is the probability of accepting a false null hypothesis. It should be small. • P1 is the proportion for group one. • P2 is the proportion for group two under the alternative hypothesis. • Odds Ratio is [P2/(1-P2)] / [P1/(1-P1)].

4. SPSS module SAMPLEPOWER • SigmaStat (SPSS product) • PASS (NCSS product) • SPLUS (Insightful product) SPSS: • www.spss.com/spssbi/samplepower/ • www.spssscience.com/SigmaStat/index.cfm NCSS: www.ncss.com/pass.html INSIGHTFUL: www.insightful.com/products/

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