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Cardiologists at Athens Medical School in Greece wanted to test whether chocolate affected blood flow in the blood vessels. The researchers recruited 17 healthy young volunteers, who were each given a 3.5-ounce bar of dark chocolate, either bittersweet or fake chocolate. On another day, the volunteers were switched. The subjects had no chocolate outside the study, and investigators didn鈥檛 know whether a subject had eaten the real or the fake chocolate. An ultrasound was taken of each volunteer鈥檚 upper arm to see the functioning of the cells in the walls of the main artery. The investigators found that blood vessel function was improved when the subjects ate bittersweet chocolate, and that there were no such changes when they ate the placebo (fake chocolate).\(^{45}\) (a) What type of design did the investigators use in their study? (b) Explain why the investigators chose this design instead of a completely randomized design. (c) Why is it important to randomly assign the order of the treatments for the subjects?

Step by step solution

01

Identify the Study Design

The description of the exercise suggests examining the effects of bittersweet versus fake chocolate using the same group of subjects in different sessions. Thus, the design is a cross-over study design, where each subject receives both treatments (bittersweet and fake chocolate) on separate occasions. The same subjects serve as their own control.

02

Compare with Completely Randomized Design

In a completely randomized design, participants would be randomly assigned to two different groups, with one group receiving the bittersweet chocolate and the other group receiving the fake chocolate. The cross-over design is more efficient in this context because it controls for individual differences in blood vessel function by comparing each subject to themselves.

03

Explain Random Assignment Importance

Randomly assigning the order of treatments helps eliminate carryover effects and bias. It ensures that the effect seen is due to the chocolate and not influenced by the order in which the treatments were received. This also helps to take into account any potential effect of time or external factors between the two treatment periods.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Cross-Over Study

In a cross-over study, each participant receives multiple treatments or interventions in a sequential manner, allowing them to act as their own control. This methodology was chosen for the chocolate study to effectively measure the impact of chocolate on blood flow. Each volunteer consumed both types of chocolate (bittersweet and fake) on different days. This design eliminates variability between subjects because the comparison is within the same individual. Therefore, differences in responses are attributed more confidently to the treatment itself. By using this method, researchers can have a clearer insight into the direct effects of both types of chocolate since confounding factors related to individual differences are minimized.

Randomized Assignment

Randomized assignment in a study refers to the process of randomly determining the order of treatments for each participant. It ensures that the study results are not biased by any systematic order of treatment. In the chocolate study, it's crucial that each person doesn't always consume real or fake chocolate first. Randomly determining the sequence in which chocolate types are assigned helps balance out possible order effects.

• For instance, if everyone had real chocolate first, any fatigue or learning effect experienced might skew results.
• It balances external variables, like daily fluctuations in conditions or moods, that might inadvertently affect the outcomes.

This randomness ensures that the findings from the study can be more confidently attributed to the effects of the chocolates, rather than the experiment's procedural influences.

Experimental Control

Maintaining experimental control involves carefully regulating the conditions of an experiment to isolate the effects of the treatment. In the described study, several measures were taken to ensure this control, particularly by preventing any chocolate consumption outside of the study. This step ensured that any observed changes in blood flow could be attributed to the chocolate eaten during the experiment, rather than any other sources.

• Also, the researchers didn鈥檛 know which chocolate the participants consumed during their testing period, which means the study was blinded. This blinding is critical because it prevents researchers' expectations from influencing the outcomes.
• Additionally, using the same ultrasound method to assess artery function ensured consistency in measurements.

This focus on control helps ensure the reliability and validity of the study's findings, emphasizing how essential experimental controls are to the integrity of experimental research.