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Chapter 19 - Two Sample t Procedures. Used when comparing two populations or treatments aka: ‘ two sample ’ problems… Samples can be of different sizes. Samples are independent (Matching violates independence). Both populations are normal.
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Chapter 19 - Two Sample t Procedures • Used when comparing two populations or treatments • aka: ‘two sample’ problems… • Samples can be of different sizes. • Samples are independent (Matching violates independence). • Both populations are normal. ex: Calcium intake vs. blood pressure change.
Diff between means = Not Significant. Example 6.9 - Chapin Social Insight Test Diff between means = Not Significant.
2 Sample Confidence Intervals Example 6.9 - Chapin Social Insight Test 0 included in interval We cannot reject Ho at the = .05 level…
(Robustness)2 • For: • equal sample sizes • similar shaped distributions • probabilities from the t table are accurate when the sample sizes are as small as (n1 & n2) = 5. • When the two distributions have different shapes, larger samples are needed. • Rule of thumb: pg 452 guidelines, replace “sample size” with “sum of sample sizes n1+n2” • Results in conservative guidelines: Option1 (software) vs. Option 2 (no software) for degrees of freedom… • Option 2 DF = (n1 - 1) OR (n2 - 1) whichever is smaller…
