Dependent t-test

Varieties of t 1-Sample (only one kind) 2-Sample (two kinds) Independent samples – groups are unrelated Experimental vs. control groups (at random) Male vs. female participants (unrelated) Dependent samples – groups are related Same person in both groups Left hand vs. right hand Pain-free vs. placebo Groups are related somehow Wife, husband Cancer patient, friend

Dependent t Formulas Observations come in pairs. Brother, sister, repeated measure. Dependent t is equivalent to a single sample t computed on differences. Problem solved by finding diffs between pairs Di=Xi1-Xi2. df=N(pairs)-1

Dependent t Example Brother (X1) Sister(X2) 5 7 8 3 Diff 2 1

Six Steps 1. Alpha = .05 2. Null & Alt: 3. Calc stat: 4. Crit value: df = N-1 =2, alpha .05, 2 tails. 5. Decision rule: 6. Conclusion: t = 4.30 Reject if |tobt| > tcrit N.S., i.e., no difference.

Faces – how attractive are they? B Rate each face on a scale from 1 to 10 where 1 is unattractive and 10 is extremely attractive.