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 50 mice were assigned to 24
hours of light (LL). Researchers observed the mice for two years, beginning
when the mice were 2 months old. Four of the LD mice
and 14 of the \(\mathrm{LL}\) 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, vol. 22 [December \(2001]: 441-447)\)
\begin{tabular}{|l|r|c|}
\hline & LD & LL \\
\hline Tumors & 4 & 14 \\
\hline No tumors & 46 & 36 \\
\hline
\end{tabular}
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?