Exam 1: Collecting Data in Reasonable Ways

For which of the following types of studies is it impossible to draw cause-and effect conclusions? I). Completely randomized experiments II)) Randomized block experiments III). Observational studies

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

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

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

Suppose you want to learn how often students of the Faculty of Arts and Sciences of Harvard University visit the gym. You interviewed students of this faculty with an athletic build. Is the sample likely to be representative of the population of interest? ​

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

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."

The entire collection of individuals or objects about which information is desired is called a sample.

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

Which of the following statistical studies is an experimental study? ​

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

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

To estimate the proportion of students who plan to purchase tickets to an upcoming school fundraiser, a high school decides to sample 100 students as they register for the spring semester. There are 2000 students at the school. Which of the following sampling plans would result in a simple random sample? ​

Random subpopulations of a population are called strata.

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

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?

Fifty-five people volunteered to participate in a statistical study. For each volunteer, a coin was tossed. If the coin landed tails up, the volunteer was assigned to group 1. If the coin landed heads up, the volunteer was assigned to group 2. Those in group 1 were asked to eat carrot salad daily for 1 year. Those in group 2 were asked not to eat carrot salad for 1 year. At the end of the year, the progress in improving vision was significantly higher for group 1 than for group 2. Did the study use a random selection from some population? Did the study use random assignment to experimental groups? ​

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.

Select the correct sampling method for the following situation. ​ To analyze the level of employee satisfaction at stores in a certain large city, a grocery store chain selects eight store locations at random and surveys all employees at these locations. ​

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