Results

ANOVA

A researcher was interested in preventing coulrophobia (fear of clowns) in children. She did an experiment in which different groups of children (15 in each) were exposed to positive information about clowns. The first group watched adverts in which Ronald McDonald is seen dancing with children and singing about how they should love their mums. A second group was told a story about a clown who helped some children when they got lost in a forest (what a clown was doing in a forest remains a mystery). A third group was entertained by a real clown, who made balloon animals for the children. A final, control, group had nothing done to them at all. Children rated how much they liked clowns from 0 (not scared of clowns at all) to 5 (very scared of clowns). Use a Kruskal–Wallis test to see whether the interventions were successful.

ANOVA - Beliefs
Cases Sum of Squares df Mean Square F p
InfoType 33.933 3 11.311 7.038 < .001
Residuals 90.000 56 1.607  
Note.  Type III Sum of Squares

Descriptives

Descriptives plots

Kruskal-Wallis Test

As seen in the output below, children’s fear beliefs about clowns was significantly affected the format of information given to them, H(3) = 17.06, p < .001. Dunn's post-hoc comparisons with Holm adjusted p-values showed that fear beliefs were significantly higher after the adverts compared to the story, z = 3.71, pholm = .001, rrb = 0.68, and exposure, z = 3.41, pholm= .003, rrb = 0.59. None of the other pairwise comparisons showed significant differences, while some substantive effect sizes were found, suggesting a higher sample size might be warranted.

Kruskal-Wallis Test
95% CI for Rank ε²
Factor Statistic df p Rank ε² Lower Upper
InfoType 17.058 3 < .001 0.289 0.135 0.567

Dunn

Dunn's Post Hoc Comparisons - InfoType
Comparison z Wi Wj rrb p pbonf pholm
Advert - Story 3.714 45.033 21.867 0.684 < .001 0.001 0.001
Advert - Exposure 3.410 45.033 23.767 0.587 < .001 0.004 0.003
Advert - None 2.197 45.033 31.333 0.667 0.028 0.168 0.112
Story - Exposure -0.305 21.867 23.767 0.044 0.761 1.000 0.761
Story - None -1.518 21.867 31.333 0.422 0.129 0.774 0.387
Exposure - None -1.213 23.767 31.333 0.356 0.225 1.000 0.450
Note.  Rank-biserial correlation based on individual Mann-Whitney tests.