Results

A psychologist was interested in the cross-species differences between men and dogs. She observed a group of dogs and a group of men in a naturalistic setting (20 of each). She classified several behaviours as being dog-like (urinating against trees and lampposts, attempts to copulate, and attempts to lick their own genitals). For each man and dog she counted the number of dog-like behaviours displayed in a 24-hour period. It was hypothesized that dogs would display more dog-like behaviours than men. Analyse the data with a Mann–Whitney test.

Independent Samples T-Test

As seen below, U = 205.5, and we had 20 men and 20 dogs. The effect size is, therefore:

r_rb = 1 − (2U)/(n1*n2) = 1 − (2 * 205.5)/(20 * 20) ≃ 0.0275.

This represents a tiny effect (it is close to zero) and the 95% confidence interval is quite wide and includes 0, which tells us that there truly isn't much difference between dogs and men., which tells us that there truly isn’t much difference between dogs and men.

We could report something like (note I’ve quoted the mean ranks for each group):

Men (mean R = 20.23) and dogs (mean R = 20.78) did not significantly differ in the extent to which they displayed dog-like behaviours, U = 205.5, p = .892, r_rb = 0.0275 (95% CI [-0.32, 0.37]).

Independent Samples T-Test
							95% CI for Rank-Biserial Correlation
	U	df	p	Hodges-Lehmann Estimate	Rank-Biserial Correlation	SE Rank-Biserial Correlation	Lower	Upper
behaviour	205.500		0.892	1.743×10^-5	0.028	0.183	-0.323	0.371

Note. For the Mann-Whitney test, effect size is given by the rank biserial correlation.
Note. Mann-Whitney U test.

Assumption Checks

Test of Normality (Shapiro-Wilk)
Residuals	W	p
behaviour	0.940	0.036

Note. Significant results suggest a deviation from normality.

Q-Q Plots

behaviour

Descriptives

Group Descriptives
	Group	N	Mean	SD	SE	Coefficient of variation	Mean Rank	Sum Rank
behaviour	Dog	20	28.050	10.981	2.455	0.391	20.775	415.500
	Man	20	26.850	9.901	2.214	0.369	20.225	404.500

Raincloud Plots

behaviour