Question

A dog food company wishes to test a new high-protein formula for puppy food to determine whether it promotes faster weight gain than the existing formula for that puppy food. Puppies participating in an experiment will be weighed at weaning (when they begin to eat puppy food) and will be weighed at one-month intervals for one year. In designing this experiment, the investigators wish to reduce the variability due to natural differences in puppy growth rates. Which of the following strategies is most appropriate for accomplishing this?
A) Block on dog breed and randomly assign puppies to existing and new formula groups within each
breed.
B) Block on geographic location and randomly assign puppies to existing and new formula groups
within each geographic area.
C) Stratify on dog breed and randomly sample puppies within each breed. Then assign puppies by
breed to either the existing or the new formula.
D) Stratify on geographic location of the puppies and randomly sample puppies within each
geographic area. Then assign puppies by geographic area to either the existing or the new formula
E) Stratify on gender and randomly sample puppies within gender groups. Then assign puppies by
gender to either the existing or the new formula.

Answer 1

Answer:

A dog food company wishes to test a new high-protein formula for puppy food to determine whether it promotes faster weight gain than the existing formula for that puppy food. Puppies participating in an experiment will be weighed at weaning (when they begin to eat puppy food) and will be weighed at one-month intervals for one year. In designing this experiment, the investigators wish to reduce the variability due to natural differences in puppy growth rates. The following strategy is the most appropriate for accomplishing it:

A) Block on dog breed and randomly assign puppies to existing and new formula groups within each  breed.

B) Block on geographic location and randomly assign puppies to existing and new formula groups  within each geographic area.

Step-by-step explanation:

Within a RTC (Randomized Block design) the investigators may categorize the puppies or the geographic location as in option B,  into blocks or dog breed or geographic subsets, as a baseline to increase statistical performance, decreasing variability within them, before assigning them randomly to the independent variable of feeding them with existing and new formula.

Options C, D and, E stratified answers refer to classifying individuals of a population into strata, instead of classifying experimental units, which leads to not having a baseline to compare, and complicate the experiment.

Answer 2

Variability refers to the degree of difference in the outcomes of an experiment from the mean value. In other to reduce variability, stratified sampling is usually employed so that each group has a proportional amount of subject in the sample. Hence, the study could employ the following options ;

• Stratify on gender and randomly sample puppies within gender groups. Then assign puppies by
• Stratify on gender and randomly sample puppies within gender groups. Then assign puppies bygender to either the existing or the new formula.

• Stratify on dog breed and randomly sample puppies within each breed. Then assign puppies by
• Stratify on dog breed and randomly sample puppies within each breed. Then assign puppies bybreed to either the existing or the new formula

The natural growth rate of puppies could be due to the development rate based on gender or breed, hence taking a stratified sample of based on these two factors, then randomly assigning each the dog's based on breed or gender to the new and existing feed formulas should reduce the variability of the outcome.

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