Sampling mall shoppers You may have seen the mall interviewer, clipboard in hand, approaching people passing by. Explain why even a large sample of mall shoppers would not provide a trustworthy estimate of the current unemployment rate in the city where the mall is located.
The sample is likely to overestimate the unemployment rate since individuals without work have more time to spend at the mall than those who do.
We have to explain why even a large sample of mall shoppers interviewed by a mall interviewer would not provide a trustworthy estimate of the current unemployment rate.
A large sample of mall shoppers would not provide a trustworthy estimate of the current unemployment rate in the city where the mall is located because persons without jobs have more time to spend at the mall than those who do, the sample is likely to exaggerate the unemployment rate.
Consider an experiment to investigate the effectiveness of different insecticides in
controlling pests and their impact on the productivity of tomato plants. What is the best
reason for randomly assigning treatment levels (spraying or not spraying) to the
experimental units (farms)?
a. Random assignment eliminates the effects of other variables, like soil fertility.
b. Random assignment eliminates chance variation in the responses.
c. Random assignment allows researchers to generalize conclusions about the
effectiveness of the insecticides to all farms.
d. Random assignment will tend to average out all other uncontrolled factors such as
soil fertility so that they are not confounded with the treatment effects.
e. Random assignment helps avoid bias due to the placebo effect.
Seat belt use A study in El Paso, Texas, looked at seat belt use by drivers. Drivers were
observed at randomly chosen convenience stores. After they left their cars, they were
invited to answer questions that included questions about seat belt use. In all, 75% said
they always used seat belts, yet only 61.5% were wearing seat belts when they pulled into
the store parking lots. Explain why the two percentages are so different.
Stronger players A football coach hears that a new exercise program will increase upper-body strength better than lifting weights. He is eager to test this new program in the off-season with the players on his high school team. The coach decides to let his players choose which of the two treatments they will undergo for weeks—exercise or weight lifting. He will use the number of push-ups a player can do at the end of the experiment as the response variable. Which principle of experimental design does the coach’s plan violate? Explain how this violation could lead to confounding.
Frozen batteries Will storing batteries in a freezer make them last longer? To find out, a company that produces batteries takes a random sample of 100 AA batteries from its warehouse. The company statistician randomly assigns 50 batteries to be stored in the freezer and the other 50 to be stored at room temperature for 3 years. At the end of that time period, each battery’s charge is tested. Result: Batteries stored in the freezer had a significantly higher average charge. What conclusion can we draw from this study? Explain your reasoning.
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