1. Psychology 202a Advanced Psychological Statistics
October 12, 2017

2. The pooled variance estimate
Weighted average of the two individual variance estimates:
df = n1+n2 - 2

3. The two-sample independent-groups t test
where

4. What’s the null hypothesis?

5. What if it doesn’t make sense to pool the variances?
Satterthwaite’s approximation for degrees of freedom:
Use unpooled variances for the standard error with adjusted degrees of freedom.
The t test in R.

6. Assumptions of the t test
Independence within each population.
Independence between populations.
Equal variances in the two populations.
Also known as “homoscedasticity.”
Both populations normally distributed.

7. Evaluating the assumptions
Independence within populations: examine the data collection procedure.
Independence between populations: examine the process that created the groups.
Random assignment guarantees independence between populations.

8. Evaluating the assumptions
Homoscedasticity:
Graphical comparisons of the two groups
Comparison of the two sample standard deviations
Normality:
Graphical examination of each group
Q-Q plots