Ch. 14--Student's T test for correlated, independent samples
1. Introduction
2.Student's T for correlated groups.
A. Correlated groups defined
i. one sample, two observations
ii. Two samples
B. Hypothesis generation for correlated group designs
C. Calculating the statistic tobt using, D
Recall for single sample:
Testing a headache drug: Two groups who have been matched on sex, age and SES are asked to participate. Group one gets a placebo and group 2 gets our new drug. We want to know if your drug reduces headache intensity. We measure headache intensity on a scale (lower scores=lower intensity). Test at alpha = .01 1 tailed.
Group 1 Group 2
98 82
96 72
140 90
120 108
130 72
125 80
110 98
D. Evaluating the statistic
3. Student's t for independent samples
A. Independent groups defined
B. Hypothesis generation for independent group designs
C. Calculating the statistic,
i. sampling distribution
Ii. tobt for independent samples
e.g.,
Group 1-given joke Group 2-no joke
18 9
16 13
21 8
14 15
17 17
19 21
22 11
24
25
18
D. Evaluating the statistic, df= n1 + n2 -2
E. tobt when sample sizes are equal
4. Assumptions in using the t test
A. correlated samples
B. independent samples
i. sampling distribution of x1- x2 is normally distributed
ii. Homogeneity of variance = the variances in the populations from which the samples means were sampled are equal
C. t is robust, especially when n > 30.