Powerandalpha

# Powerandalpha - but with a.47 risk of a false alarm/Type I...

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More concepts about inferential testing: How does alpha level relate to (and differ from) statistical signiFcance level? How do I know how much power I have? What is enough power? Monday, October 10, 2011

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Statistical Significance levels • Alpha level: –The selected maximum risk of Type I Error that is acceptable • Probability value (p-value): –The smallest probability of a Type I Error you could have adopted and still rejected the null hypothesis. Monday, October 10, 2011
One-Sample Statistics N Mean Std. Deviation Std. Error Mean bloodsugar 10 -2.0000 8.21922 2.59915 One-Sample Test -.769 9 .461 -2.00000 -7.8797 3.8797 bloodsugar t df Sig. (2-tailed) Mean Difference Lower Upper 95% Confidence Interval of the Difference Test Value = 0 Monday, October 10, 2011

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• Suppose df=9, and observed t = .769, and you have a more thorough table of critical values for the t-test. df .50 .40 .30 .20 .10 .05 1 2 9 .70-ish .80-ish 1.0-ish 1.383 1.833 2.262 An observed t-test statistic: .769 If you had adopted alpha = ~.47, you would have rejected your null hypothesis,

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Unformatted text preview: but with a .47 risk of a false alarm/Type I error. Alpha levels Monday, October 10, 2011 Power and Sample size • The actual amount of power in a study depends on where the H1 distribution is actually located: Monday, October 10, 2011 Null Hypothesis True Null false/Alternative Hypothesis True Reject H0 Fail to Reject H0 Critical Value Monday, October 10, 2011 7 Type II error is more likely Type I error risk is still equal to alpha Here, between-groups variability in the population is small, so the effect size is relatively small. Smaller effect size means less POWER. Monday, October 10, 2011 Power and Sample Size Handout • Two usages: –(1) Estimate the power in an already completed analysis • “adequate” power is considered .80 –(2) Plan an appropriate N, given effect size estimate from prior research 8 Monday, October 10, 2011...
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## This note was uploaded on 10/26/2011 for the course SLL 330 taught by Professor Damare during the Spring '10 term at USC.

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Powerandalpha - but with a.47 risk of a false alarm/Type I...

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