In 1964, Ehrlich and Raven proposed that plant chemical defenses against attack by herbivores would spur plant diversification. In a test of this idea (Farrell et al. 1991), the number of species in 16 pairs of sister clades whose species differed in their level of chemical protection were counted.

In each pair of sister clades, the plants of one clade were defended by a gooey latex or resin that was exuded when the leaf was damaged, whereas the plants of the other clade lacked this defense. In 13 of the 16 pairs, the clade with latex/resin was found to be the more diverse (had more species), whereas in the other 3 pairs the sister clade lacking latex/resin was found to be the more diverse.

a. With these data, test whether the clade having latex/resin and its sister clade lacking it are equally likely to be more diverse.

b. Is this a controlled experiment or an observational study? Why?