In this lab assignment you will learn how to examine data produced in a simple s

Question

In this lab assignment you will learn how to examine data produced in a simple statistical experiment. In particular, you will examine the experiment design and apply graphical, numerical, and inferential tools available in StatCrunch to compare two distributions produced by the study. Before you start working on the assignment, you should review the course material about designing experiments and comparing two population means. Caffeine Dependence Experiment Caffeine is the world's most widely consumed mood-altering substance. In North America, about 90% of adults consume caffeine daily. Coffee is the leading dietary source of caffeine among adults in Canada, while soft drinks represent the largest source of caffeine for children. People who consume large amounts of caffeine each day may experience physical withdrawal symptoms if they stop taking in their usual amounts of the substance. In this lab assignment you will follow an experiment on caffeine dependency conducted on a group of volunteers by researchers from Johns Hopkins University School of Medicine, in Baltimore and summarized in the paper “Caffeine Dependence Syndrome. Evidence from Case Histories and Experimental Evaluations”, JAMA (The Journal of the American Medical Association), Vol. 272, No. 13, 1994. The researchers recruited twenty-seven volunteers who believed they were psychologically or physically addicted to caffeine but otherwise were in good health. Of these twenty-seven volunteers, sixteen were diagnosed as caffeine dependent based on some general substance dependence criteria. Of the sixteen subjects who were diagnosed as caffeine dependent, eleven agreed to participate in a study to evaluate their caffeine dependency. Before the experiment was conducted, daily caffeine intake measurements for each subject were obtained based on food diaries of the participants. The experiment was conducted on two 2-day periods which occurred exactly one week apart. During one of the 2-day periods, the subjects were given a set of capsules containing the amount of caffeine normally ingested by the subject in one day. During the other study period, the subjects were given placebos. The order in which each subject received the two types of capsules was randomized. At the end of each 2-day study period, subjects were evaluated in three areas: depression symptoms, fatigue and vigor. The experimenters were blinded to whether the subject was receiving the caffeine pills or the caffeine-free pills. The data are available in the StatCrunch file lab4.txt located on the STAT 151 Laboratories website at http://www.stat.ualberta.ca/statslabs/stat151/index.htm (click Stat 151 link, and Data for Lab 4). The data are not to be printed in your submission. The following is a description of the variables in the data file: Variable Name Description of Variable SUBJECT Subject number (a whole number from 1 to 11), DEPR-CAF Depression score during caffeine period, DEPR-NC Depression score during no-caffeine period, FATIGUE-CAF Fatigue score during caffeine period, FATIGUE-NC Fatigue score during no-caffeine period, VIGOR-CAF Vigor score during caffeine period, VIGOR-NC Vigor score during no-caffeine period, SMOKER Smoker status (Y if smoked cigarettes daily, N otherwise), CAFFEINE Daily intake of caffeine (in mg). 2 Use the data provided to answer the following questions: 1. First you will examine the experiment design. (a) Can we treat the 11 subjects who agreed to participate in the study as a random sample from the population of all caffeine dependent individuals? Explain why or why not? Can you generalize the results of the study to the population of all caffeine dependent individuals? Explain briefly. (b) Why were both the subjects and the experimenters interviewing the subjects blinded to whether the subject was receiving the caffeine pills or the caffeine-free pills? (c) Why was the order in which the two series of capsules were taken randomized? (d) Why were the two study periods held one week apart instead of using two consecutive 2-day periods?

data

Subject Depr-Caf Depr-NC Fatig-Caf Fatig-NC Vigor-Caf Vigor-NC Smoker Caffeine
1 5 16 5 28 23 1 Y 2548
2 5 23 16 20 25 12 N 231
3 4 5 9 5 9 11 Y 642
4 3 7 3 23 13 2 N 1029
5 8 14 0 4 17 10 N 302
6 5 24 1 27 30 2 N 430
7 0 6 5 13 15 2 N 342
8 0 3 3 2 19 21 Y 295
9 2 15 0 28 28 5 N 371
10 11 12 7 17 13 2 N 270
11 1 0 25 7 4 21 N 516

Explanation / Answer

c) The variables DEPR-CAF, DEPR-NC, FATIGUE-CAF, FATIGUE-NC, VIGOR-CAF, VIGOR-NC depend both on physical conditions as well as psychological conditions. If the subjects know what capsules they are given, their psychological conditions might change accordingly. It may happen that, if they know they are taking placebos, they might feel energetic just because they know they are not consuming any caffiene and the data may not give the actual effect of caffiene in their bodies.

d) The two study periods were held one week apart instead of using two consecutive 2-day periods. Beacuse if two consecutive 2-day periods were used, the data may not suggest the real effect of caffiene on the subject's body. By using two periods one week apart,after the first study period the subject's pyschological and physical conditions should go back to normal in one week time before the second study period held and then it will reflect the true effect of the capsules.

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