Question

a researcher is studying the effect of genetically modified (gm) and nongenetically modified (ngm) com on the weight gain of lambs. the sex and genetics of the lambs can affect their weight gain. five sets of male twin lambs and five sets of female twin lambs - for a total of twenty lambs - are available for the study. the lambs will be randomly assigned to a diet of either gm or ngm diet of corn. weight gain will be recorded for each lamb after five weeks on the diet. which of the following designs would be best to use in the study?
a a completely randomized design. randomly assign ten lambs to the gm diet and ten lambs to the ngm diet.
b a stratified randomized design. divide the lambs into males and females. within each group, randomly assign the gm diet to one half and the ngm diet to the other half.
c a randomized block design. randomly assign ten lambs to the gm diet and ten lambs to the ngm diet.
d a randomized block design. randomly assign the gm diet to the male lambs and the ngm diet to the female lambs.

Explanation:

• A completely randomized design (Option A) ignores the fact that sex can affect weight gain. This could lead to confounding variables.
• A stratified randomized design (Option B) is appropriate. Since sex (male/female) is a known factor that can influence weight gain (a confounding variable), stratifying by sex (dividing into males and females) and then randomly assigning treatments (GM and nGM diets) within each stratum controls for the effect of sex.
• Option C is incorrect as it is similar to Option A in not accounting for the variable of sex.
• Option D is incorrect because randomly assigning diets based on sex (all males to one diet and all females to another) is not a proper random assignment within homogeneous groups. It does not use the twin - set structure and may introduce bias.

Answer:

B. A stratified randomized design. Divide the lambs into males and females. Within each group, randomly assign the GM diet to one half and the nGM diet to the other half.