Deck 2: Collecting Data Sensibly

Question 1

Random assignment to treatments will guarantee groups that are exactly alike for experimental purposes.

Answer 1

False

Question 2

In a well-designed experiment, the factors are confounded whenever possible.

Answer 2

False

Question 3

A treatment is any particular combination of values for the explanatory variables.

Answer 3

True

Question 4

As long as the sample size is small relative to the population, there is little practical difference between sampling with replacement and sampling without replacement.

Question 5

Increasing sample size will generally eliminate bias in a sample.

Question 6

Clusters are non-overlapping subgroups of a population that have been identified as homogeneous.

Question 7

Blocking is a technique that can be used to filter out the effects of extraneous factors.

Question 8

A study is an observational study if the investigator observes the behavior of a response variable when one or more factors are manipulated.

Question 9

A control group provides a baseline for comparison to a treatment group.

Question 10

Stratified sampling is a sampling method that in no way involves simple random sampling.

Question 11

Selection bias can occur if volunteers only are used in a study.

Question 12

Random subpopulations of a population are called strata.

Question 13

A placebo is identical in appearance to the treatment of interest, but contains no active ingredients.

Question 14

Random assignment of volunteers should result in comparable experimental groups.

Question 15

By definition, a simple random sample of size n is any sample that is selected in a manner to guarantee every individual in the population has an equal chance of selection.

Question 16

Response bias can occur when responses are not actually obtained from all individuals selected for inclusion in the sample.

Question 17

Two factors are extraneous if their effects on the response variable cannot be distinguished from one another.

Question 18

If the subjects as well as the person measuring the response are aware of the treatment assigned to the subject, only single-blinding is being used.

Question 19

The method of control wherein an extraneous variable is held constant is called blocking.

Question 20

Replicating in an experiment means that the number of subjects is greater than 1.

Question 21

A friend of yours, who is not taking statistics, wonders why it is that anyone would choose to take a sample. "Obviously," she says, "you would get better information from a census." In a short paragraph, explain why it is that statisticians take samples rather than taking a census.

Question 22

The following paragraph describes an actual study. After reading the description, determine whether the study is an observational or experimental study. Justify your answer with specific references to the information in the study. "Before the opening of the new International Airport and the termination of the old airport, children near both sites were recruited into aircraft-noise groups (aircraft noise at present or pending) and control groups with no aircraft noise and closely matched for socio-economic status. A total of 326 children (mean age = 10.4 years) took part in three data-collection waves, one before and two after the switch-over of the airports. After the switch to the new airport, long-term memory and reading were found to be impaired in the noise group at the new airport, and improved in the formerly noise-exposed group at the old airport."

Question 23

Bias, the tendency for samples to differ from the corresponding population in some systematic way, might be due to: (a) selection bias, (b) response bias, and/or (c) nonresponse bias. In a few sentences, discuss the differences among these different biases.

Question 24

Suppose that two experiments were conducted to compare the effects of two new insect repellants. In Experiment X, a simple random sample was taken from the population of River City, and the citizens were randomly assigned to repellant A and repellant B. In Experiment Y, a stratified random sample was taken from the population of River City, and the citizens were randomly assigned to repellant A and repellant B. (River City has 4 geographic areas of town, the NE, NW, SE, and SW quadrants; 100 people from each quadrant were sampled for Experiment B.) The results of the experiments were the same: fewer insects landed on the arms that had been treated with insect repellant A. (a) For each experiment, X and Y, discuss whether one can legitimately infer from each of these experiments that repellant A is better than B? Why or why not? (b) For each experiment, X and Y, discuss whether one can legitimately generalize to the population of River City from each of these experiments? Why or why not?

Question 25

Suppose that two experiments were conducted to assess the effect of a new insect repellant. In Experiment A, a simple random sample was taken from the population of River City. In Experiment B, a simple random sample from a group of volunteers from the population of River City was used. The results of the experiments were the same: fewer insects landed on the arms that had been treated with the insect repellant. The volunteers were randomly assigned to the two treatments in both experiments. (a) For each experiment, A and B, discuss whether one can legitimately infer a cause-and-effect relation between the use of the repellant and fewer insects landing from each of these experiments? Why or why not? (b) For each experiment, A and B, discuss whether one can legitimately generalize to the population of River City from each of these experiments? Why or why not?

Question 26

A pharmaceutical company wants to test its new drug that is designed to help balding men grow more hair. From their records of past customers, the company has data on about 5,000 men. The data contains information about the men's hair color, age, and percent of baldness. (A partial list is given below.) For their anticipated experiment, they want to take a sample that is representative of their customers. (a) Briefly describe how you would select a simple random sample of size n = 20 from this list of customers. (b) Describe in a short paragraph why you might wish to use a stratified random sample.

Question 27

One of the tasks of wildlife biologists is to estimate bird populations. One method for doing this is to walk a "transect," typically a long randomly selected rectangle, and count the number of particular species that are seen. During hunting season the biologists wear brightly colored clothes as a protective measure. A theory, known as the "species-confidence hypothesis," predicts that birds may react to these brightly colored clothes. A robin, for example, when confronted with a biologist wearing an orange color may be less afraid than if the biologist were wearing a brown color. The idea is that the orange color gives the robin some "confidence" that the biologist is another (really big!) bird. This is a difficulty because a robin that is less afraid of the biologist may be easier to see and therefore "count" because the robin lets the biologist get closer. This would result in too high an estimate of the robin population. To test this theory a biologist performed a study in her area, wearing a bright orange vest or a brown vest while working. After training herself to accurately estimate distances she indicated on her notes how close the robins would let her get before they flew away (the "approach distance"), using this as a measure of their "confidence." She randomly selected her starting points in each trip through the transects and also carefully noted the weather conditions, specifically the wind speed. (a) What is the explanatory variable (factor) for this experiment? (b) What is the response variable for this experiment? (c) After completing the study the biologist was examining her results and noticed that on those days she wore the brightly-colored vest the wind tended to be of greater velocity. How does this information affect the interpretation of the results of the experiment?

Question 28

Luis being a college student has undertaken some research. The results have shown that people who buy expensive cars are less likely to die from a flu-related illness during the following year than those who do not buy. However, recent research has shown that people who buy expensive cars are also less likely to die from any cause during the following year than those who don't buy. Are there any confounding variables that make it difficult to justify a cause-and-effect conclusion? ​

A) No, there are no confounding variables that make it difficult to justify a cause-and-effect conclusion.
B) Genetic inheritance of a flu-related illness is the only confounding variable.
C) General health, home environment, socio-economic status, or wealth - are examples of possible confounding variables.
D) Addiction to buying expensive cars is the only confounding variable.

Question 29

A common practice of teachers is to have students exchange their quizzes and grade each others. In addition to decreasing the teacher's work load, the reduced time between quiz and feedback is thought to be a plus for learning. Your U.S. History teacher, aware of your statistical prowess, has asked you to design an experiment to test this theory. You have decided to use the mid-term exam (not graded by students) as your response measure. Your history teacher has three classes, one early in the morning, one at noon, and one late in the afternoon. Each class contains 30 students. (a) Describe the treatments you will use in your experiment (b) One possible confounding variable is the time of day, since students may be more alert at certain times of the day than at other times. Describe a method would you use to control this variable? (Unfortunately you cannot change the student schedules!) (c) Do you feel the results of your experiment could be generalized to your statistics class? Why or why not?

Question 30

In evaluating an experiment, how would you determine if a variable is an explanatory variable or a response variable?

Question 31

The following paragraph describes an actual study. After reading the description, determine whether the study is an observational study or an experiment. Justify your answer with specific references to the information in the study. "We compared paired daytime and night counts of wild brook trout, brown trout, and rainbow trout made by the same snorkelers in five streams during August 1994. Overall, we counted 109 trout in the daytime and 333 trout at night. We speculate that trout counted at night were present during the daytime but were hidden from view. Biologists should consider that trout behavior and susceptibility to being seen might vary a great deal between daytime and night, even during summer. In some streams, the majority of trout may not be seen during the daytime."

Question 32

The two paragraphs below discuss aspects of two studies, each of which exhibit a bias. For each study, decide whether the problem is selection bias, response bias, or nonresponse bias, and in a few sentences explain why you chose your answer. (a) One part of the Nurses' Health Study is concerned with possible causes of skin cancer. Nurses were asked about different behaviors and aspects of their health when they entered the study. Then, the nurses were given the questionnaire again if they were diagnosed with cancer. When the questionnaires were analyzed, the investigators discovered that after the nurses were diagnosed with cancer they tended to report a reduced ability to tan. It is thought that the shift in reporting might be caused by an awareness of their diagnosis. (b) One part of the Demographic and Health Surveys Program is concerned with measures of malnutrition. Investigators measure physical aspects of growing children, and attempt to document the physical characteristics of a population at different ages. Sadly, in some countries many children die early, and thus a bias is introduced in the study when the investigators can not collect the data from the deceased children.

Question 33

The ZZZ chain of motels has a standard method of constructing their rooms to maximize the ease of parking for its customers. The rooms are arranged in adjacent buildings so that each customer can park outside the rented room. The layout for one of the with 48 rooms located along a famous highway is diagrammed below: Route 66 The manager would like to survey customers in 12 of his rooms (one randomly selected customer for each room selected in the sample) to assess their satisfaction with the motel services. The surveys will be placed on the customers' beds before they check in to the motel. In order to make the directions easy to follow, he elects to use systematic sampling. (a) Explain how you would use random numbers to set up the systematic sampling process. (b) Write a short paragraph for the maids that helps them carry out your method in part (a).

Question 34

Three methods for random sampling are: (a) simple random sampling, (b) stratified random sampling, and (c) cluster sampling. In a few sentences, discuss the similarities and differences among these sampling methods. Specifically, what sampling circumstances would lead you to choose each of these methods?

Question 35

In evaluating an experiment, how would you determine if a variable is an explanatory variable or an extraneous variable?

Question 36

In competitive sports, video recorders have been used more frequently in recent years. The idea behind the recorder is that coaches can replay training sessions for more effective feedback to the athlete. Some people believe video recording may make the athletes more nervous and actually decrease their performance. You have been asked to design an experiment to address this issue for competitive high school tennis players. You have decided to use the accuracy of tennis serves as your response variable, and the number of successful serves out of 100 as your performance measure. The subjects for your experiment are 60 high school male competitive tennis players of varying ability who have volunteered for the experiment. (a) Describe the treatments in your experiment (b) One possible confounding variable is the experience levels of the players. Explain how you would control this variable? (c) Can the results of this experiment could be generalized to all male tennis players? Why or why not?

Question 37

We have distinguished two types of studies: observational and experimental. Briefly explain the essential difference(s) between these two types of study.

Question 38

Studies made by Kate as a university student have shown that people over age 60 who donate blood twice a year are less likely to die from a heart disease during the following year than those who do not donate. However, recent research has shown that people over age 60 who donate blood twice a year are also less likely to die from any cause during the following year than those who don't donate. Are there any confounding variables that make it difficult to justify a cause-and-effect conclusion?

A) Yes
B) No

Question 39

The likelihood that an animal will successfully avoid a predator's attack should increase if the presence of the predator is detected. Animals are, of course, known to warn others of a predator's presence--the well-known prairie-dog warning call is an example. Some aquatic animals are known to send chemical signals. For example the crayfish and Iowa darter may excrete ammonium from their gills or possibly in their urine during periods of heightened metabolic activity. In an experiment to see if red-legged frogs (Rana aurora) may send or respond to chemical signals, specimens in Oregon were collected as embryos and raised in an aquarium. As tadpoles they were separated into two pairs of aquaria. The "upper tank" in each pair was connected to the "lower tank" to create a (very!) small river. A wooden heron model was placed in one of the upper tanks and moved around for 30 seconds to simulate a predator attack. Both the sender (upper tank) and receiver (lower tank) animals were monitored for added activity (increased movement) indicating antipredator behavior. (a) What is the explanatory variable (factor) for this experiment? (b) What is the response variable for this experiment? (c) During the course of the experiment the investigators were very careful with the wooden heron model not to come in contact with the glass of the aquaria or make noise in any other way. If they had been unsuccessful and their wooden heron made significant amounts of noise, how would that affect the interpretation of the results?