Power and Sample Size

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Power and Sample Size. Boulder 2006 Benjamin Neale. I HAVE THE POWER!!!. To Be Accomplished. Introduce concept of power via correlation coefficient ( ρ ) example Identify relevant factors contributing to power Practical: Empirical power analysis for univariate twin model (simulation) - PowerPoint PPT Presentation

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  • Power and Sample SizeBoulder 2006Benjamin Neale

  • To Be AccomplishedIntroduce concept of power via correlation coefficient () exampleIdentify relevant factors contributing to powerPractical:Empirical power analysis for univariate twin model (simulation)How to use mx for power

  • Simple exampleInvestigate the linear relationship (r)between two random variables X and Y: r=0 vs. r0 (correlation coefficient).

    draw a sample, measure X,Y calculate the measure of association r (Pearson product moment corr. coeff.) test whether r 0.

  • How to Test r 0assumed the data are normally distributeddefined a null-hypothesis (r = 0)chosen a level (usually .05)utilized the (null) distribution of the test statistic associated with r=0t=r [(N-2)/(1-r2)]

  • How to Test r 0Sample N=40r=.303, t=1.867, df=38, p=.06 =.05As p > , we fail to reject r = 0

    have we drawn the correct conclusion?

  • = type I error rate probability of deciding r 0(while in truth r=0) a is often chosen to equal .05...why?

    DOGMA

  • N=40, r=0, nrep=1000 central t(38), a=0.05 (critical value 2.04)

  • Observed non-null distribution (r=.2) and null distribution

  • In 23% of tests of r=0, |t|>2.024 (a=0.05), and thus draw the correct conclusion that of rejecting r = 0.

    The probability of rejecting the null-hypothesis (r=0) correctly is 1-b, or the power, when a true effect exists

  • Hypothesis TestingCorrelation Coefficient hypotheses:ho (null hypothesis) is =0 ha (alternative hypothesis) is 0Two-sided test, where > 0 or < 0 are one-sidedNull hypothesis usually assumes no effectAlternative hypothesis is the idea being tested

  • Summary of Possible ResultsH-0 trueH-0 falseaccept H-01-abreject H-0 a1-b

    a=type 1 error rateb=type 2 error rate1-b=statistical power

  • Rejection of H0Non-rejection of H0H0 trueHA true

  • PowerThe probability of rejection of a false null-hypothesis depends on: the significance criterion ()the sample size (N) the effect size ()The probability of detecting a given effect size in a population from a sample of size N, using significance criterion

  • Talpha 0.05Sampling distribution if HA were trueSampling distribution if H0 were truePOWER: 1 - Standard CaseNon-centrality parameter

  • Talpha 0.05Sampling distribution if HA were trueSampling distribution if H0 were truePOWER: 1 - Increased effect sizeNon-centrality parameter

  • Impact of more conservative Talpha 0.01Sampling distribution if HA were trueSampling distribution if H0 were truePOWER: 1 - Non-centrality parameter

  • Impact of less conservative Talpha 0.10Sampling distribution if HA were trueSampling distribution if H0 were truePOWER: 1 - Non-centrality parameter

  • Talpha 0.05Sampling distribution if HA were trueSampling distribution if H0 were truePOWER: 1 - Increased sample sizeNon-centrality parameter

  • Effects on Power RecapLarger Effect SizeLarger Sample SizeAlpha Level shifts Type of Data:Binary, Ordinal, ContinuousMultivariate analysisEmpirical significance/permutation

  • When To Do Power Calculations?Generally study planning stages of studyOccasionally with negative resultNo need if significance is achievedComputed to determine chances of success