9.72 Eggshell quality in laying hens. Introducing calcium into a hen's diet can improve the shell quality of the eggs laid. One way to do this is with a limestone diet. In Animal Feed Science and Technology (June 2010), researchers investi- gated the effect of hen's age and limestone diet on eggshell quality. Two different diets were studied–fine limestone (FL) and coarse limestone (CL). Hens were classified as either younger hens (24–36 weeks old) or older hens (56–68 weeks old). The study used 120 younger hens and 120 older hens. Within each age group, half the hens were fed a fine limestone diet and the other half a coarse limestone diet. Thus, there were 60 hens in each of the four combinations of age and diet. The characteristics of the eggs produced from the laying hens were recorded, including shell thickness. a. Identify the type of experimental design employed by the researchers. b. Identify the factors and the factor levels (treatments) for this design. c. Identify the experimental unit. d. Identify the dependent variable. e. The researchers found no evidence of factor interac- tion. Interpret this result, practically. f. The researchers found no evidence of a main effect for hen's age. Interpret this result, practically. g. The researchers found statistical evidence of a main ef- fect for limestone diet. Interpret this result, practically. (Note: The mean shell thickness for eggs produced by hens on a CL diet was larger than the corresponding mean for hens on an FL diet.)