Transcript Probability - University of Central Missouri

POST HOC COMPARISONS
What is the
Purpose?
How Do Post Hoc
Methods Differ?
How Does the
Tukey’s HSD Work?
What is the Purpose?
A significant F in ANOVA tells you only
that there is a difference among the
groups, not which groups are different.
 Post hoc tests can be done to make
specific comparisons.
 Post hocs also control experimentwise
alpha level.

How Do Post Hoc
Methods Differ?
Some post hocs are more conservative,
meaning careful about avoiding Type I
error.
 Post hocs methods also differ in what
type of comparisons they allow

 Pairwise: one condition compared to another
 Nonpairwise: combinations of conditions
How Do Post Hoc
Methods Differ?

Commonly used post hoc methods:
 Fisher’s PLSD
 Scheffe
 Tukey’s HSD
Fisher’s PLSD
Protected Least Significant Difference
 Less conservative
 F-test in ANOVA must be significant
 Pairwise comparisons only

Scheffe
Very conservative
 Pairwise or nonpairwise comparisons
 High critical value in exchange for
allowing any comparison

Tukey’s HSD



Medium conservative
Pairwise comparisons only
Each group must have same n
How Does the Tukey’s HSD Work?
Honestly Significant Difference is
calculated.
 Any difference between means must be
larger than the HSD to be significant.
 Larger differences are needed as more
groups are being compared; this
controls Type I error rate.
