Researchers conducted a double-blind, placebo-controlled, repeated-measures experiment to compare the effectiveness of a commercial caffeinated carbohydrate–electrolyte sports drink with a commercial noncaffeinated carbohydrate–electrolyte sports drink and a flavored-water placebo. Sixteen highly trained cyclists each completed three trials of prolonged cycling in a warm environment: one while receiving the placebo, one while receiving the noncaffeinated sports drink, and one while receiving the caffeinated sports drink. For a given trial, one beverage treatment was administered throughout a 2-hour variable-intensity cycling bout followed by a 15-minute performance ride. Total work in kilojoules (kJ) performed during the final 15 minutes was used to measure performance. The beverage order for the individual subjects was randomly assigned. A period of at least 5 days separated the trials. All trials took place at approximately the same time of day in an environmental chamber at 28.5°C and 60% relative humidity with fan airflow of approximately 2.5 meters per second (m/s).

The researchers found that cycling performance, as assessed by the total work completed during the performance ride, was 23% greater for the caffeinated sports drink than for the placebo and 15% greater for the caffeinated sports drink than for the noncaffeinated sports drink. Cycling performances for the noncaffeinated sports drink and the placebo were not significantly different. The researchers concluded that the caffeinated carbohydrate–electrolyte sports drink substantially enhanced physical performance during prolonged exercise compared with the noncaffeinated carbohydrate–electrolyte sports drink and the placebo.

(a) What does it mean for the experiment to be placebo-controlled?

(b) What does it mean for the experiment to be double-blind?

Why do you think it is necessary for the experiment to be double-blind?

(c) How is randomization used in this experiment?

(d) What is the population for which this study applies? What is the sample?

(e) What are the treatments?

(f ) What is the response variable?

(g) This experiment used a repeated-measures design, a design type that has not been directly discussed in this textbook.

Using this experiment as a guide, determine what it means for the design of the experiment to be repeated-measures. How does this design relate to the matched-pairs design?