Skip to content

Quiz: Experimental Design

Test your understanding of experimental design principles including treatments, control groups, randomization, replication, and blinding with these review questions.

1. What is the primary purpose of random assignment in an experiment?

  1. To ensure the sample represents the population
  2. To create groups that are similar in both known and unknown variables
  3. To increase the sample size
  4. To eliminate the need for a control group
Show Answer

The correct answer is B. Random assignment distributes both known and unknown confounding variables approximately equally across treatment groups. This is different from random sampling, which helps with generalization. Random assignment is the key to establishing causation because it makes groups comparable before treatment is applied.

Concept Tested: Random Assignment

2. In an experiment testing three different fertilizers on plant growth, the experimental units are the individual plants. What are the treatments?

  1. The three different fertilizers
  2. The plants receiving each fertilizer
  3. The measured plant growth
  4. The type of soil used
Show Answer

The correct answer is A. Treatments are the specific experimental conditions applied to the experimental units. In this case, each of the three fertilizers is a different treatment. The plants are the experimental units (what receives the treatment), and plant growth is the response variable being measured.

Concept Tested: Treatment

3. Why do experiments include a placebo control group?

  1. To reduce the cost of the experiment
  2. To account for improvement that occurs simply from believing one is receiving treatment
  3. To ensure random assignment is properly conducted
  4. To increase the number of treatments being tested
Show Answer

The correct answer is B. The placebo effect causes real, measurable changes in people simply because they believe they are receiving treatment. A placebo control group receives an inactive treatment that appears identical to the real treatment. By comparing the treatment group to the placebo group, researchers can isolate the actual effect of the treatment beyond psychological effects.

Concept Tested: Placebo Effect

4. An experiment has two factors: temperature (3 levels) and humidity (2 levels). How many different treatments are there?

  1. 2
  2. 5
  3. 6
  4. 3
Show Answer

The correct answer is C. The number of treatments equals the product of the levels of all factors. With 3 temperature levels and 2 humidity levels, there are 3 x 2 = 6 different treatment combinations. Each unique combination of factor levels constitutes a separate treatment condition.

Concept Tested: Levels of a Factor

5. In a double-blind experiment, who does NOT know which treatment participants are receiving?

  1. Only the participants
  2. Only the researchers administering treatments
  3. Both the participants and the researchers interacting with them
  4. Only the statisticians analyzing the data
Show Answer

The correct answer is C. In a double-blind experiment, neither the subjects nor the researchers who interact with them know which treatment is being administered. This prevents both participant expectation effects and unconscious researcher bias from influencing results. A separate team typically knows the assignments for safety purposes.

Concept Tested: Double-Blind Experiment

6. Which experimental design pairs similar subjects together, then randomly assigns one member of each pair to each treatment?

  1. Completely randomized design
  2. Randomized block design
  3. Matched pairs design
  4. Stratified experimental design
Show Answer

The correct answer is C. A matched pairs design creates pairs of similar experimental units (or uses each subject as their own control) and randomly assigns treatments within each pair. This controls for individual variation and is especially powerful when comparing exactly two treatments. It's a special case of blocking with block size of 2.

Concept Tested: Matched Pairs Design

7. A researcher blocks subjects by age group before randomly assigning them to treatments. What is the main advantage of this approach?

  1. It eliminates the need for random assignment
  2. It reduces variability by controlling for a known source of differences
  3. It increases the total sample size needed
  4. It allows the researcher to choose which subjects get which treatment
Show Answer

The correct answer is B. A randomized block design groups subjects by a characteristic (like age) that might affect the response variable, then randomly assigns treatments within each block. This ensures each treatment group has equal representation from each age group and allows researchers to separate the effect of age from the treatment effect.

Concept Tested: Randomized Block Design

8. Why is replication important in experimental design?

  1. It allows researchers to avoid using control groups
  2. It reduces the impact of individual variation and helps detect real treatment effects
  3. It eliminates the need for random assignment
  4. It ensures the experiment can be published
Show Answer

The correct answer is B. Replication means using enough experimental units in each treatment group. With sufficient replication, individual variation averages out, making it easier to detect true treatment effects. One or two subjects per group could show differences due to chance; many subjects per group provide convincing evidence that observed differences are real.

Concept Tested: Replication

9. What is the key difference between random sampling and random assignment?

  1. Random sampling is used in experiments; random assignment is used in surveys
  2. Random sampling selects who participates; random assignment determines which treatment each participant receives
  3. Random sampling requires larger sample sizes than random assignment
  4. Random sampling and random assignment are the same thing
Show Answer

The correct answer is B. Random sampling determines who is selected from a population to participate in a study, helping ensure the sample represents the population (external validity). Random assignment determines which treatment each participant receives, helping establish causation by creating comparable groups (internal validity). An ideal experiment uses both.

Concept Tested: Why Randomize

10. An experiment tests whether a new teaching method improves test scores. Students can see which method they're using, but the person grading the tests doesn't know which method each student used. What type of blinding is this?

  1. No blinding
  2. Single-blind
  3. Double-blind
  4. Triple-blind
Show Answer

The correct answer is B. This is a single-blind experiment because one party (the grader) is blind to treatment assignment while another party (the students) is not. Since students cannot be blind to whether they're using a new teaching method, the researchers blind the outcome assessment instead. This prevents grading bias from affecting results.

Concept Tested: Single-Blind Experiment