Psychology 202a Advanced Psychological Statistics

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Psychology 202a Advanced Psychological Statistics October 12, 2017

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

The two-sample independent-groups t test where

What’s the null hypothesis?

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.

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.

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.

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

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