91Ó°ÊÓ

Two factors helpful? A two-factor experiment designed to compare two diets and to analyze whether results depend on gender randomly assigns 20 men and 20 women to the two diets, 10 of each to each diet. After three months the sample mean weight losses are as shown in the table. Caffeine jolt A study (Psychosomatic Medicine 2002 ; \(64: 593-603)\) claimed that people who consume caffeine regularly may experience higher stress and higher blood pressure. In the experiment, 47 regular coffee drinkers consumed 500 milligrams of caffeine in a pill form (equivalent to four 8 -oz cups) during one workday, and a placebo pill during another workday. The researchers monitored the subjects' blood pressure and heart rate, and the subjects recorded how stressed they felt. a. Identify the response variable(s), explanatory variable, experimental units, and the treatments. b. Is this an example of a completely randomized design, or a crossover design? Explain.

Step by step solution

01

Identify Response Variable(s)

The response variables are the outcomes that are being measured. In this exercise, the response variables are the blood pressure and heart rate of the subjects, along with their self-reported stress levels.

02

Identify Explanatory Variable

The explanatory variable is the variable manipulated by the researcher to observe its effect on the response variables. Here, the explanatory variable is the consumption of caffeine in pill form versus a placebo pill.

03

Identify Experimental Units

The experimental units are the individuals or items that are being studied. In this experiment, the experimental units are the 47 regular coffee drinkers who participate in the study.

04

Determine the Treatments

The treatments are the different conditions applied to the experimental units. Here, the treatments consist of consuming a caffeine pill (500 milligrams of caffeine) and consuming a placebo pill.

05

Classify the Design

This is a crossover design because each subject receives both treatments (caffeine and placebo) on different occasions, and their responses are compared.

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Key Concepts

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

Response Variable

In any experiment, the response variable is crucial as it represents the outcome or the effect that the researchers are interested in measuring. For the study regarding caffeine consumption, the response variables are the participants' blood pressure and heart rate, as well as their self-reported stress levels. These are the dependent variables since they are presumed to change in response to the manipulation of other variables. By carefully measuring these outcomes, researchers can assess the impact of the independent variable on these health indicators.

Explanatory Variable

The explanatory variable is what the experimenter manipulates to determine if it causes any changes in the response variables. In this particular study, the explanatory variable is the type of pill consumed by the participants – either a caffeine pill or a placebo. By changing this variable, researchers aim to ascertain its direct impact on the response, which includes physiological metrics like blood pressure and psychological aspects like stress levels. Understanding the role of the explanatory variable is key in establishing causation versus correlation in experimental analysis.

Crossover Design

A crossover design is a type of experimental setup where participants are exposed to more than one treatment in a sequential manner. In this caffeine study, each subject receives both the caffeine pill and the placebo pill on different days. This design is advantageous because it allows each participant to serve as their own control, thereby reducing variability in the experiment. The crossover method leads to improved accuracy in understanding how a single variable—caffeine consumption in this scenario—affects health metrics. This design's strength lies in its ability to isolate the effects of the treatments more effectively than simple parallel-group designs.

Treatments

In experimental terms, treatments refer to the specific conditions applied to experimental units. For the caffeine experiment, the two treatments involved either ingesting a caffeine pill containing 500 milligrams of caffeine or taking a placebo pill. Treatments are central to experimental design as they provide the means by which researchers can test their hypotheses about the explanatory variable. By strictly controlling and documenting treatments, experimenters can make valid inferences about the effects of these conditions on the response variables.

Experimental Units

The experimental units in research are the subjects or entities observed or experimented upon. In the caffeine study, the experimental units are the 47 regular coffee drinkers who took part in the experiment. Each participant's data contributes individually to the results, providing a comprehensive look at how caffeine affects the specified health outcomes. Understanding the role of experimental units helps in structuring an experiment as well as in data collection, ensuring that observations are systematically and consistently documented across all subjects.