W5 - A priori and post hoc comparisons

Multiple comparisons

Conducting more than one statistical comparison in a study, which increases the risk of Type I error

Per comparison approach

Treats the Type I error rate separately for each individual test

Familywise approach

Considers the probability of making at least one Type I error across all comparisons combined

Familywise error rate

The overall probability of making one or more Type I errors across a set of comparisons

Post-hoc comparison

Comparisons decided after examining the data, often involving all possible pairs and requiring error correction

A priori comparison

Comparisons planned before data collection based on hypotheses, usually fewer and more focused

Linear contrasts

A method using weighted combinations of group means to compare one group or set of groups with another

Psi

The symbol used to represent the value of a linear contrast

Orthogonal contrasts

Independent contrasts that do not overlap in information and whose weights satisfy specific conditions

Bonferroni t

A correction method that divides the familywise alpha by the number of comparisons to control error rate

Dunn test

Another name for the Bonferroni correction used for multiple comparisons

Error rate per comparison

The probability of making a Type I error in a single test

Tukey’s test

A post-hoc test that compares all possible pairs of means while controlling the familywise error rate

Pairwise comparison

A comparison between two group means at a time

Keppel’s recommendation

A guideline suggesting no alpha adjustment if planned comparisons do not exceed degrees of freedom, and a modified Bonferroni adjustment if they do