Cellulose from the butts of smoked cigarettes is commonly used by urban birds in nest construction. In an observational study of the housefinches in Mexico City, Suárez-Rodríguez et al. (2013) discovered that nests with more cellulose from smoked cigarette butts contained fewer nest-dwelling ectoparasites of birds (such as mites) than nests with less cellulose from smoked cigarettes. In a separate experimental study, the researchers placed thermal traps in 28 active house finch nests (the parasites hiding in the nest are drawn to the warmth and become trapped). Smoked Marlboro cigarette butts were placed in the trap in about half the nests, randomly chosen. In the other nests, filters from unsmoked cigarettes (lacking tobacco residues) were used as control. At the end of the experiment, the researchers counted the number of ectoparasites caught in each trap. The traps containing smoked butts had fewer ectoparasites than traps with unsmoked filters.

a. Which study provided stronger evidence that the chemical contents of smoked cigarette butts deter ectoparasites: the observational study or the experimental study? Explain your reasoning.

b. Which of the six commonly used components of experimental design were not incorporated in the experimental study described above? What benefit might result from including them?