Question 1 A professor conducted a study on the effect of the amount of caffeine on anxiety. Of the 60 subjects available for the study, 20 were randomly assigned to Group 1 (giving 2 cups of decaffeinated coffee with 0mg caffeine), another 20 were randomly assigned to Group 2 (giving 2 cups of coffee with 2mg caffeine), and the remaining 20 were assigned to Group 3 (giving 2 cups of coffee with 6mg caffeine). After 30 minutes of consumption of the three types of coffee mixtures by the three different groups each of the subjects were given a test that measures their level of perceived anxiety. (a) (2 marks) Is the above study observational or experimental? In less than 50 words clearly explain your choice based on the extract given above. The study is experimental. Unlike in observational studies where variables of interests are measured without allocating treatments to the subjects, in experimental studies, treatments are applied to the subjects. In this study, people are grouped and treated (given coffee); hence, the study is experimental. (b) (2 marks) Identify and write down the name of the response variable(s) and explanatory variable(s) of interest. The response variable in the experiment is the level of perceived anxiety while the explanatory variable is the amount of caffeine. (c) (3 marks) How many levels of the explanatory variable are in the study? Clearly describe all the levels. The levels of explanatory variables refer to the number of experimental conditions. In this study, there are three levels of the explanatory variables: i) Group 1: Decaffeinated coffee ii) Group 2: 2mg caffeine iii) Group 3: 6 mg of caffeine (d) (2 marks) Explain why, or why not, Group 1 can be treated as a ‘placebo’ group in the context of the study. Group 1 can be treated a placebo group because the members of the group are treated with inert (decaffeinated coffee) substance. This allows the outcomes of Group 2 and Group 3, those treated STA2300 Data Analysis, Semester 3 2019, 0061032987 with active substance (caffeine), to be compared with those of the placebo group (Group 1) to examine whether the investigational treatment in more effective than in Group 1. (e) (4 marks) Explain which principles of experimental design were implemented, and how, in the study? The principles of experimental design used in this study are local control and randomisation. Randomisation is achieved by using a random process to assign experimental units. In this experiment, the 60 subjects assigned to the three groups (each 20 subjects) are randomly assigned, hence achieving randomisation. The principle of local control is achieved by including a group in which subjects are treated. In this experiment, Group 1 subjects are not given caffeine, hence acting as a placebo group and achieving the principle of local control. (f) (3 marks) Explain what is meant by an experimental unit. Identify the experimental units in the context of the study. How many experimental units are used in the study? Experimental unit refers to the physical entity in an experiment in which treatment is allocated at random. Accordingly, an experimental unit refers to units of statistical analysis in an experiment. In this study, experimental units are the groups. Given that the study has three groups, there are three experimental units in the study.
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