1. Identify the population and the sample.
Warm Up
Identify the population and the sample.
1. Fifty teenagers are randomly selected and asked whether they have siblings.
population: teenagers; sample: the 50 selected teenagers
2. Fifteen passengers on a flight are asked about the quality of the flight attendants’ service.
population: passengers on the flight; sample: the 15 people asked

2. 3. To determine how many cats have blue eyes, the eye colors of 10 cats at each of 10 animal shelters in different states are recorded.
population: cats; sample: the 100 cats whose eye colors are recorded

3. Objectives
Focus on the commonalities and differences between surveys, experiments, and observational studies.

4. Vocabulary experiment observational study controlled experiment
control group
treatment group
randomized comparative

5. You have already seen that a survey is one way to collect data
You have already seen that a survey is one way to collect data. Although surveys are useful, different situations require different techniques for gathering data.
Individuals are people, animals, or objects that are described by data. If you collect data on the fuel efficiency of cars and trucks, the individuals are vehicles. Variables are used to describe individuals. Fuel efficiency, measured in miles per gallon, is an example of a variable.

7. For data from an experiment to be useful, the experiment must be carefully designed. In a controlled experiment, two groups are studied under conditions that are identical except for one variable. The effects of the treatment are determined by comparing the control group and the treatment group.

8. Often, to demonstrate a cause and effect hypothesis, an experiment must show two things. First, that a phenomenon occurs with the treatment; and second, that the phenomenon does not occur in the absence of the treatment.
In a randomized comparative experiment, the individuals are assigned to the control group or the treatment group at random, in order to minimize bias. An experiment that is not a randomized comparative experiment may be subject to bias, and any conclusions drawn from the experiment may not be valid.

9. Example 1: Identifying Experiments and Observational Studies
Explain whether each situation is an experiment or an observational study.
A. A researcher wants to know if a soil additive makes a fern grow more quickly. He grows one specimen in treated soil and one in untreated soil.
The researcher applies a treatment, so the situation is an experiment.
B. To find out whether car accidents are more likely on rainy days, a researcher records the weather conditions during 50 randomly selected accidents for the past year.

10. Continued Example 1: Identifying Experiments and Observational Studies
The researcher gathers data without controlling the individuals or applying a treatment. The situation is an example of an observational study.

11. Check It Out! Example 1
A scientist measures the height of 20 birds’ nests, and counts the number of eggs to see if there is a relationship. Is this experiment or an observational study? Explain.
It is an observational study because an observation
is being conducted without controlling the environment in any way

12. Example 2: Evaluating a Published Report
The study described in the report is a randomized
comparative experiment. Describe the treatment, the treatment group, and the control group.
The treatment is bathing in Epsom salt. The treatment group consists of the fifty subjects who
bathe in the Epsom salt, and the control group consists of the fifty subjects who did not.

13. Check It Out! Example 2
The study described in the report is a randomized
comparative experiment. Describe the treatment, the treatment group, and the control group.
The treatment is using the old and new websites to make purchases. The treatment group consists of users of the website using the new website to make purchases. The control group consists of users of the website using the old website to make purchases

14. Example 3: Designing an Experiment or Observational Study
Explain whether the research topic is best addressed through an experiment or an observational study. Then explain how you would set up the experiment or the observational study.
The treatment may affect health, so it is not ethical to assign individuals to a treatment group. Perform an observational study. Randomly choose one group of
people who already use tanning beds at least twice a month and another group which does not. Monitor the incidence of skin diseases in both groups.

15. Check It Out! Example 3
Explain whether the research topic is best addressed through an experiment or an observational study. Then explain how you would set up the experiment or the observational study.
1000 mg of vitamin is not known to have any negative effects, so conduct an experiment. Randomly choose one group of people to take the supplements, randomly choose another group of
people to not take the supplements, and monitor the cholesterol levels in both groups.

16. Example 4: Evaluating Data Collection Methods
Classify each method as a survey, an experiment, or observational study, and explain which would be most reliable.
Method A:
Method B:
Method C:
Choose 50 people who have at least one serving of soy a day and 50 who don’t, and check their cholesterol levels.
Randomly choose 100 people. Ask how many servings of soy they have a week, and ask if their cholesterol levels are high.
Randomly choose 50 people to eat at least one serving of soy a day, and 50 people not to, and monitor their cholesterol levels.

17. Continued Example 4: Evaluating Data Collection Methods
Method A: observational study
Method B: survey
Method C: experiment
In Method A, the members of the study are self-selected. They may have a healthy diet in general. In Method B, the results are self-reported, so they may not be accurate. Method C is most reliable.

18. Check It Out! Example 4
Classify each method as a survey, an experiment, or an observational study, and explain which would be most reliable.

19. Check It Out! Example 4
Method A: experiment
Method B: survey
Method C: observational study
Method B is least reliable, because there is no basis for comparison. Method C has a comparison group, but the members are self-selected, which could lead to bias. In method A, the members of each group are randomly selected, so method A is the most reliable.