notes 37 - Comparing Two Independent Means - Unpooled and...

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Comparing Two Independent Means - Unpooled and Pooled Submitted by gfj100 on Wed, 11/11/2009 - 13:41 We determine whether to apply "pooled" or "unpooled" procedures by comparing the sample standard deviations. RULE OF THUMB: If the larger sample standard deviation is MORE THAN twice the smaller sample standard deviation then perform the analysis using unpooled methods. Example 1 (Unpooled): Cholesterol levels are measured for 28 heart attack patients (2 days after their attacks) and 30 other hospital patients who did not have a heart attack. The response is quantitative so we compare means. It is thought that cholesterol levels will be higher for the heart attack patients, so a one-sided alternative hypothesis is used. Step 1: null is H 0 : μ 1 − μ 2 = 0 and alternative is H a : μ 1 − μ 2 > 0, where groups 1 and 2 are heart attack and control groups, respectively. Minitab Output that can be used for Steps 2-5 Step 2: test statistic is given in last line of output as t = 6.15, degrees of freedom given as 37.
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notes 37 - Comparing Two Independent Means - Unpooled and...

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