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Researchers wish to know if there is a link between hypertension (high blood pressure) and consumption of salt. Past studies have indicated that the consumption of fruits and vegetables offsets the negative impact of salt consumption. It is also known that there is quite a bit of person-to-person variability as far as the ability of the body to process and eliminate salt. However, no method exists for identifying individuals who have a higher ability to process salt. The U.S. Department of Agriculture recommends that daily intake of salt should not exceed 2400 mg. The researchers want to keep the design simple, so they choose to conduct their study using a completely randomized design. (a) What is the response variable in the study? (b) Name three factors that have been identified. (c) For each factor identified, determine whether the variable can be controlled, cannot be controlled, or should be manipulated. If a factor cannot be controlled, what should be done to reduce variability in the response variable? (d) How many treatments would you recommend? Why?

Step by step solution

01

Identify the Response Variable

The response variable is the primary variable that the researchers are interested in measuring to see the effect of different treatments. In this study, the researchers are investigating the link between hypertension and salt consumption, so the response variable is 'levels of hypertension' or 'blood pressure levels'.

02

Identify the Factors

Factors are the variables that might affect the response variable. In the given scenario, three identified factors are: 1) Consumption of salt, 2) Consumption of fruits and vegetables, 3) Person-to-person variability in processing and eliminating salt.

03

Determine Control for Each Factor

For each identified factor, determine if it can be controlled, cannot be controlled, or should be manipulated: 1) Consumption of salt - This can be controlled by setting specific salt intake levels for the participants. 2) Consumption of fruits and vegetables - This can also be controlled by setting specific amounts and types of fruits and vegetables to be consumed. 3) Person-to-person variability in processing salt - This cannot be controlled directly since there is no method to identify individuals with different salt processing abilities. To reduce variability due to this factor, randomization should be used to ensure an equal distribution of this variability across treatment groups.

04

Determine the Number of Treatments

The number of treatments corresponds to the different combinations of factor levels that will be tested. Since the researchers wish to use a completely randomized design and considering the factors identified: 1) Different salt intake levels (e.g., low, medium, high) 2) Different fruit and vegetable intake levels (e.g., low, recommended, high) An intersection of these levels provides a range of treatments. For simplicity, recommend four treatments: 1) Low salt, recommended fruits and vegetables 2) Medium salt, recommended fruits and vegetables 3) High salt, recommended fruits and vegetables 4) Recommended salt (based on USDA guideline), recommended fruits and vegetables. This approach keeps the study design simple while addressing the main factors of concern.

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Key Concepts

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

Response Variable

The response variable is key in any research study. It refers to the primary measure of interest that researchers want to examine. In the exercise, the response variable is 'levels of hypertension' or 'blood pressure levels'. This is because the researchers want to see how different levels of salt consumption impact blood pressure. Simply put, the response variable is what you measure in the end to determine the effect of the experimental treatments.

Factors and Treatments

Factors are the variables that can influence the response variable. In this study, three main factors have been identified:

• Consumption of salt: The amount of salt intake could affect blood pressure. This can be controlled by setting specific salt intake levels for the participants.
• Consumption of fruits and vegetables: These may offset the negative impact of salt. This can also be controlled by prescribing certain amounts and types of these foods.
• Individual variability in salt processing: Different people process salt differently, which affects their blood pressure. This factor cannot be controlled directly because there's no method to identify such individuals.

By manipulating these factors, different treatments can be created. For instance:

• Low salt, recommended fruits and vegetables
• Medium salt, recommended fruits and vegetables
• High salt, recommended fruits and vegetables
• Recommended salt (based on USDA guideline), recommended fruits and vegetables

Each combination represents a different treatment.

Randomization

Randomization is a technique used to distribute the effects of uncontrolled variables evenly across different treatment groups. This ensures that the experiment is fair and unbiased. In this study, person-to-person variability in salt processing cannot be directly controlled. Here’s where randomization helps. By assigning participants randomly to different treatment groups, researchers can make sure that this variability is evenly spread out. This minimizes its impact on the final results, so the conclusions drawn are more reliable. Randomization promotes fair comparison and improves the internal validity of the study.

Control Variables

Control variables are aspects of an experiment that are kept constant to ensure that the results are due to the treatments alone and not some other external factors.

• Salt intake: The amount of salt given to each group needs to be precisely controlled.
• Fruits and vegetables intake: Similar control for fruit and vegetable intake ensures that differences in blood pressure are due to salt levels, and not inconsistencies in other dietary factors.

In the context of this study, controlling these variables means ensuring that every participant follows the prescribed daily intake exactly. If these control variables are properly managed, it will be easier to attribute changes in the response variable to the actual treatments.