Post hoc comparisons

Question 1

what are post hoc comparisons

Answer 1

Unplanned comparisons

Used for exploratory research with no clear hypotheses

Question 2

Benefits of post hoc comparisons

Answer 2

:) useful for testing many hypotheses

:) usually more powerful than Bonferoni’s t-test (for a large number of tests)

:) robust against non-normality

Question 3

Negatives of post hoc comparisons

Answer 3

:( Post hocs are generally more conservative so have less power- there is a trade off between controlling for Type 1 errors (low alpha), and statistical power.
If the alpha is too conservative there is the risk of loosing power (Type 2 error- less chance to find an effect)

:( poor for unequal group sizes

:( poor if homogeneity of variance is violated

Question 4

Which test for equal sample sizes and homogeneity?

Answer 4

Tukey’s HSD or Bonferonni

Question 5

Which test for unequal sample sizes?

Answer 5

Hochberg’s GT2

Question 6

Which test for heterogeneity?

Answer 6

Games-Howell procedure

Question 7

Post-hoc contrasts in factorial designs

Answer 7

(more than one IV)

T Trends
I Interaction contrasts
S Simple main effects
S Simple interaction effects