Practise identifying experiments from assigned treatments, then name the experimental units, factors, treatment levels, and response variables in each study context.
A dermatologist will conduct an experiment to investigate the effectiveness of a new drug to treat acne. The dermatologist has recruited 36 pairs of identical twins. Each person in the experiment has acne and each person in the experiment will receive either the new drug or a placebo. After each person in the experiment uses either the new drug or the placebo for 2 weeks, the dermatologist will evaluate the improvement in acne severity for each person on a scale from 0 (no improvement) to 100 (complete cure).
Each twin in the experiment has a severity of acne similar to that of the other twin. However, the severity of acne differs from one twin pair to another.
Identify the treatments, experimental units, and response variable of the experiment.
- Treatments:
- Experimental units:
- Response variable:
Treatments: New drug, placebo.
Experimental units: The 72 people who receive the new drug or placebo.
Response variable: Improvement in acne severity
Essentially correct (E) if the response satisfies the following three components:
1. Identifies the treatments as new drug and
placebo
2. Identifies the experimental units as the 72 people (subjects, participants, twins) in the
experiment
3. Identifies the response variable as the
improvement in acne severity
Partially correct (P) if the response satisfies only two of the three components.
Incorrect (I) if the response does not satisfy the
criteria for E or P.
Additional Notes:
- To satisfy component 1, identification of the treatments must include both the placebo and the new drug.
- To satisfy component 2, the response must indicate that the experimental units are individual people. The response could refer to participants, subjects, twins, or members of the pairs of twins without explicitly mentioning the number 72. However, a response that states or implies that there are 36 experimental units (e.g., "the pairs of twins") does not satisfy component 2.
- To satisfy component 3, the response must include the context of "acne" and "improvement" (e.g., "improvement in acne severity," "acne improvement score"), but it does not need to include a reference to the scale, the dermatologist, two-week time periods, or treatments. Reasonable synonyms for improvement can be used, such as using "reduction" or "change" or by including the verbal descriptions of the scale ("no improvement" to "complete cure"). However, a description of a binary outcome (e.g., "whether or not the acne improves") does not satisfy component 3.
- For responses that indicate the 36 pairs of twins are the experimental units, component 3 may be satisfied by indicating that the response variable is the improvement in acne severity or by indicating that the response variable is the difference in improvement in acne severity.
- If the response provides parallel solutions (i.e., two or more complete solutions without choosing or indicating which is to be scored), the response is scored based on the weaker of the two solutions. For example, if a response says that the experimental units are "the 72 participants and the scores from 0 to 100," component 2 is not satisfied.