The consumption of caffeine to benefit alertness is a common activity practiced by 90% of adults in North America. Often caffeine is used in order to replace the need for sleep. One recent study1 compares students' ability to recall memorized information after either the consumption of caffeine or a brief sleep. A random sample of 35 adults (between the ages of 18-39 ) were randomly divided into three groups and verbally given a list of 24 words to memorize. During a break, one of the groups takes a nap for an hour and a half, another group is kept awake and then given a caffeine pill an hour prior to testing, and the third group is given a placebo. The response variable of interest is the number of words participants are able to recall following the break. The summary statistics for the three groups are in the table below. We are interested in testing whether there is evidence of a difference in average recall ability between any two of the treatments. Thus we have three possible tests between different pairs of groups: Sleep vs Caffeine, Sleep vs Placebo, and Caffeine vs Placebo.
Group
Sample size
Mean
Standard Deviation
Sleep
12
15.25
3.3
Caffeine
12
12.25
3.5
Placebo
11
13.70
3.0
1 Mednick, Cai, Kanady, and Drummond, "Comparing the benefits of caffeine, naps and placebo on verbal, motor and perceptual memory", Behavioural Brain Research, 193 (2008), 79-86.
Warning
H0.
In the sample , which group had better recall ability?
A new study1 shows that relationship status on Facebook matters to couples. The study included 58college-age heterosexual couples who had been in a relationship for an average of 19 months. In 45 of the 58 couples, both partners reported being in a relationship on Facebook. In 31 of the 58 couples, both partners showed their dating partner in their Facebook profile picture. Men were somewhat more likely to include their partner in the picture than vice versa. However, the study states: "Females' indication that they are in a relationship was not as important to their male partners compared with how females felt about male partners indicating they are in a relationship." Using a population of college-age heterosexual couples who have been in a relationhip for an average of 19 months:
(a) A 95% confidence interval for the proportion with both partners reporting being in a relationship on Facebook is about 0.66 to 0.88. What is the conclusion in a hypothesis test to see if the proportion is different from 0.5? What significance level is being used?
(b) A 95% confidence interval for the proportion with both partners showing their dating partner in their Facebook profile picture is about 0.40 to 0.66 . What is the conclusion in a hypothesis test to see if the proportion is different from 0.5? What significance level is being used?
1 Roan, Shari, "The true meaning of Facebook's 'in a relationship'", Los Angeles Times, February 23, 2012, reporting on a study in Cyberpsychology, Behavior, and Social Networking.
Chapter 5, Section 1, Exercise 013
Find the specified areas for a normal density.
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The distribution of sample proportions of US adults with a college degree for random samples of size n=500 is N(0.275,0.02). How often will such samples have a proportion, Ùp, that is more than 0.300?
A statistics instructor designed an exam so that the grades would be roughly normally distributed with mean m=75 and standard deviation δ=11. Unfortunately, a fire alarm with ten minutes to go in the exam made it difficult for some students to finish. When the instructor graded the exams, he found they were roughly normally distributed, but the mean grade was 60 and the standard deviation was 18. To be fair, he decides to ‘‘curve” the scores to match the desired N(75,11) distribution. To do this, he standardizes the actual scores to z-scores using the N(60,18) distribution and then ‘‘unstandardizes” those z-scores to shift to N(75,11).
What is the new grade assigned for a student whose original score was 45?
A survey of 1000 air travelers1 found that 60% prefer a window seat. The sample size is large enough to use the normal distribution, and a bootstrap distribution shows that the standard error is SE=0.015.
Use a normal distribution to find a 90% confidence interval for the proportion of air travelers who prefer a window seat.
Hospital admissions for asthma in children younger than 15 years was studied1 in Scotland both before and after comprehensive smoke-free legislation was passed in March 2006. Monthly records were kept of the annualized percent change in asthma admissions, both before and after the legislation was passed. For the sample studied, before the legislation, admissions for asthma were increasing at a mean rate of 5.2% per year. The standard error for this estimate is 0.7% per year. After the legislation, admissions were decreasing at a mean rate of 18.2% per year, with a standard error for this mean of 1.79%. In both cases, the sample size is large enough to use a normal distribution.
1Mackay, D., et. al., ‘‘Smoke-free Legislation and Hospitalizations for Childhood Asthma,” The New England Journal of Medicine, September 16, 2010; 363(12):1139-45.
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(a) Find a 95% confidence interval for the mean annual percent rate of change in childhood asthma hospital admissions in Scotland before the smoke-free legislation.
A study1 of 138 penalty shots in World Cup Finals games between 1982 and 1994 found that the goalkeeper correctly guessed the direction of the kick only 41% of the time. The article notes that this is ‘‘slightly worse than random chance.” We use these data as a sample of all World Cup penalty shots ever. Test at a 5% significance level to see whether there is evidence that the percent guessed correctly is less than 50%. The sample size is large enough to use the normal distribution. The standard error from a randomization distribution under the null hypothesis is SE=0.043.
1St.John, A., ‘‘Physics of a World Cup Penalty-Kick Shootout - 2010 World Cup Penalty Kicks,” Popular Mechanics, June 14, 2010.
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Reject H0 and find evidence that the proportion guessed correctly is not less than half.
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Reject H0 and find evidence that the proportion guessed correctly is less than half.
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Do not reject H0 and do not find evidence that the proportion guessed correctly is less than half.
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Do not reject H0and find evidence that the proportion guessed correctly is less than half.
Chapter 5, Section 2, Exercise 065
How Often Do You Use Cash?
In a survey1 of 1000 American adults conducted in April 2012, 43% reported having gone through an entire week without paying for anything in cash. Test to see if this sample provides evidence that the proportion of all American adults going a week without paying cash is greater than 40%. Use the fact that a randomization distribution is approximately normally distributed with a standard error of SE=0.016. Show all details of the test and use a 5% significance level.
1‘‘43% Have Gone Through a Week Without Paying Cash,” Rasmussen Reports, April 11, 2011.
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