PSYC227_fall09_week10

PSYC227_fall09_week10 - Factors that Influence a Hypothesis...

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Factors that Influence a Hypothesis Test 1. Size of the mean difference 2. The variability of the scores 3. The number of scores in the sample
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Population: normal distribution with µ =80, σ =10. M = 81, α =0.05 n = 25 n =400 Effect size Cohen’s d = (81 – 80)/10 = .1 1.96 0.025 0.025 2 0.5 =1
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Effect size Magnitude Evaluation of Effect Size 0 < d < 0.2 small effect (mean difference less than 0.2 SD) 0.2 <d < 0.8 medium effect d > 0.8 large effect
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Statistical Power The power of a statistical test is the probability that the test will correctly reject a false null hypothesis . It is the probability that the test will identify a treatment effect if one really exists.
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M=83.92 µ σ
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M=83.92 0 P(z > -2.04) = 0.9793 The power of the test is 97.93% z
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M=83.92 0 P(z > -2.04) = 0.9793 0.9793 The power of the test is 97.93%
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Sample size Alpha level z=2.58 for α =0.01 One-tailed vs. two-tailed tests z=1.65 for one- tailed test Factors that affect power Previous example, n=25 here, n=4
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Example The average grade for a seniors at the local high school is 78 , with a standard deviation of 12 . The distribution is normal. A researcher would like to evaluate the effectiveness of a new study-skills training program using a sample of 16 students. The researcher plans to examine the results using a two-tailed hypothesis test with alpha 0.05 . 1.
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This note was uploaded on 02/24/2011 for the course PSYC 227 taught by Professor Shinkareva during the Fall '09 term at South Carolina.

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PSYC227_fall09_week10 - Factors that Influence a Hypothesis...

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