It is an ongoing debate, what properties of visualizations increase people’s
performance when solving Bayesian reasoning tasks. In the discussion of the
properties of two visualizations, i.e., the tree diagram and the unit square, we
emphasize how both visualizations make relevant subset relations transparent.
Actually, the unit square with natural frequencies reveals the subset relation that
is essential for the Bayes’ rule in a numerical and geometrical way whereas
the tree diagram with natural frequencies does it only in a numerical way.
Accordingly, in a first experiment with 148 university students, the unit square
outperformed the tree diagram when referring to the students’ ability to
quantify the subset relation that must be applied in Bayes’ rule. As
hypothesized, in a second experiment with 143 students, the unit square was
significantly more effective when the students’ performance in tasks based on
Bayes’ rule was regarded. Our results could inform the debate referring to
Bayesian reasoning since we found that the graphical transparency of nested sets
could explain these visualizations’ effect.