Question

To test whether two different skin creams A and B have different effects on the human body, $n$ persons were enrolled in a clinical trial. Then cream A was applied to one arm and cream B to the other arm of each of the $n$ randomly selected persons. What type of design is this?

• A. Completely Randomized Design
• B. Randomized Block Design
• C. Latin Square Design
• D. Balanced Incomplete Design

Hint:

Whenever you see a "before and after" study or a "left arm vs. right arm" study, it is almost always a Randomized Block Design because the subject acts as their own control (block).

Answer 1

The Correct Option is B

We analyze the experimental structure to identify the appropriate statistical design.

Step 1: \color{redIdentify the Experimental Units
The experimental units are the arms of the persons.

Step 2: \color{redIdentify the Nuisance Variable
The variation between different people (skin type, age, health) is a major source of noise that could mask the difference between the creams.

Step 3: \color{redAnalyze the Grouping (Blocking)
By applying both treatments (Cream A and Cream B) to the same person, we are using the "person" as a block. This allows us to compare the effects of the creams within each individual, effectively controlling for the person-to-person variation.

Step 4: \color{redConclusion
A design where experimental units are grouped into homogeneous sets (blocks) and treatments are assigned randomly within those blocks is a Randomized Block Design (RBD). In this specific case, it is often called a "matched pairs" design, which is the simplest form of RBD.
Final Answer: (2).