Effects of Light Exposure A study carried out by Baturin and colleagues looked at the effects of light on female mice.

Fifty mice were randomly assigned to a regimen of 12 hours of light and 12 hours of dark (LD), while another fifty mice were assigned to 24 hours of light (LL). Researchers observed the mice for two years, beginning when the mice were two months old. Four of the LD mice and 14 of the LL mice developed tumors. The accompanying table summarizes the data.

(Source: Baturin et al., The effect of light regimen and melatonin on the development of spontaneous mammary tumors in mice, Neuroendocrinology Letters, 2001)

a. Determine the percentage of mice that developed tumors from each group (LL and LD). Compare them and comment.

b. Was this a controlled experiment or an observational study? How do you know?

c. Can we conclude that light for 24 hours a day causes an increase in tumors in mice? Why or why not?