Sample Size

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2SampleSizeThe rstquestionfacedbya statisticalconsultant,andfrequentlythela st,is, Howmany subjects(animals,units)doI need? , TypeII error, andone sidedversustwo questionistosettlethetypeofvariable(endp oint)theconsulteehasinmind:Isit continuous,discrete,orsomethingelse?Forc ontinuousmeasurementsthenormaldistributi onisthedefaultmodel,fordistributionswith binaryoutcomes, samplesizecalculation,foroneortwo groups,are:TypeI Error( )Probabilityofrejectingthenullhypothesis whenit is trueTypeII Error( )Probabilityofnotrejectingthenullhypothe siswhenit is falsePower=1 Probabilityofrejectingthenullhypothesisw henit is false 20and 21Variancesunderthenullandalternative hypothe ses(maybethesame) 0and 1Meansunderthenullandalternative hypothesesn0andn1Samplesizesintwo groups(maybethesame)Thechoiceofthealtern ative hypothesisis they knew thevalueofthealternative hypothesis,they wouldnotneedtodothestudy.

CALCULATING SAMPLE SIZE USING THE COEFFICIENT OF VARIATION 31 Lakatos (1998). One advantage of a power of 0.95 is that it bases the inferences on

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