3.18 In the 1990s, scientists became concerned about worldwide declines in the population sizes for amphibians. It was thought that one of the possible causes was the increasing amount of ultraviolet (UV) light to which the eggs were exposed, as many amphibians lay their eggs in shallow water fully exposed to sunlight. An experiment was designed for a shallow water creek in Oregon that is home to three amphibians, the toad, the tree frog, and the Cascade frog, which possess differing levels of an enzyme that protects eggs against the effects of UV light. Two types of UV filters were used, a blocking filter and a transmitting filter, along with a control group involving no filter (Ramsey and Schafer, The Statistical Sleuth, 1997).

a Describe the factors and levels for this experiment.

How many treatments are there?

b The experimental units were 36 enclosures set up in the shallow water, each with room for 150 eggs. Describe how you would randomize the treatments to the experimental units.