Question

A drug made by Company A is used to promote growth in chickens is being compared to two different generic competitors made by Companies B & C. The drug at its intended dose can be delivered to the birds in either feed or in water. Eight different rooms each containing at least 7 floor pens were used for the study and fifty 1-day-old chicks were placed into each pen on the same day for each room. Each of the 6 combinations of manufacturer and delivery method were randomly assigned to one pen per room. The 7th pen in each room received no drug in its feed or water. At the end of the study, the total weight (in pounds) of all birds in the same pen was measured using a pen scale, i.e. birds were weighed together as a group.

a) Describe the treatment structure of the experiment including the treatment factor(s) and their corresponding levels.

b) Identify the experimental unit(s) and the observational unit. Specify the total number of (each size of) experimental units used in the experiment.

c) Identify the design structure and any factors associated with the design structure (e.g. blocking factors).

d) Classify the response on the OU as nominal, ordinal, interval or ratio.

Answer 1

Let us understand the design of this experiment first -

Treatment - Company whose drug is used AND mode of delivery

Response - Growth (total weight of the birds)

Experimental Unit - Room and Floor Pens

Observational Unit - Pens as the grouping is done at Pen level.

Treatment levels are basically number of changes to the independent variable. Here treatment factor "Company whose drug is used" has 3 levels - A, B and C.

The other treatment factor "Mode of delivery" has two levels - Feed or in water.

Classify the response -

A ratio variable, has all the properties of an interval variable, and also has a clear definition of 0.0. When the variable equals 0.0, there is none of that variable. For example, because weight is a ratio variable, a weight of 4 grams is twice as heavy as a weight of 2 grams.