Logic of Causal Inference in Controlled Experiments

A researcher wants to test the effectiveness of a new public speaking training program. They allow the first 40 volunteers who sign up to participate in the new program (the experimental group). The next 40 volunteers are assigned to a waitlist and will serve as the control group. The researcher finds that the experimental group performs significantly better on a final speaking task. Why is it difficult to conclude that the training program caused the improvement?

Problem of Systematic Differences in Experiments

What is the primary purpose of using random assignment in an experimental design?

In an experiment with 200 participants, a researcher uses random assignment to place participants into either a meditation group or a control group. A colleague argues that random assignment guarantees the two groups will have identical average ages, stress levels, and personality traits before the study begins. Is this claim accurate?

Dr. Patel is conducting an experiment to test whether a new study technique improves test scores. Arrange the steps of her research process in the logical sequence that illustrates how random assignment ensures group comparability and allows for causal conclusions.

Researchers use random assignment to ensure their experimental results are valid and groups are comparable. Match each research situation with the specific role or outcome of random assignment it illustrates.

You are designing a study to test whether a new digital flashcard app improves vocabulary retention in $200$ college students. To ensure that your experimental and control groups are comparable regarding their baseline memory, motivation, and prior vocabulary knowledge before the study begins, which of the following procedures should you develop?

Random assignment in experimental design distributes participant characteristics evenly across groups to minimize pre-existing systematic differences, ensuring the groups are comparable before the study begins.

In a psychological experiment, random assignment plays a vital role in ensuring that group comparisons are valid. Match each key aspect of this process with the explanation of its role in establishing group comparability.

When evaluating the internal validity of an experiment with a very small sample, a researcher may be skeptical of causal claims because random assignment is less likely to have successfully ensured group _____, leaving the study vulnerable to the influence of pre-existing participant differences.

In a between-subjects experiment, a researcher analyzes potential confounds and notes that random assignment distributes participant characteristics evenly. This process minimizes systematic differences to ensure group _____ before the study begins, allowing any post-treatment differences to be attributed to the independent variable.

Arrange the steps of an experimental evaluation process in the correct logical order to demonstrate how a researcher establishes group comparability to confidently attribute outcome differences to the independent variable.

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

Because its role is to ensure group comparability, random assignment guarantees that there will be absolutely no pre-existing differences between participants in the experimental and control groups.

Dr. Lin is designing an experiment to test a new exercise program. Match each of her potential participant assignment strategies with its resulting impact on group comparability.

Analyze the logical progression of experimental design. Arrange the following steps in the correct order to demonstrate how random assignment fulfills its role in ensuring group comparability and enabling researchers to draw valid causal conclusions.

A journal editor evaluates an experimental manuscript and decides to reject it because the researchers assigned early-arriving participants to the treatment group and late-arriving participants to the control group. The editor correctly justifies this rejection by stating that the lack of random assignment failed to ensure initial group ____, meaning any observed differences could simply be due to pre-existing participant characteristics.

In experimental design, random assignment minimizes the likelihood of pre-existing systematic differences between groups. By ensuring this initial comparability, researchers can confidently attribute any observed outcome differences to which of the following?

Match each term related to experimental design with its correct description concerning how random assignment ensures group comparability.

Dr. Ahn is designing an experiment to test whether a new software interface improves data-entry speed. She has a sample of 120 data-entry clerks who vary in their typing experience, typical accuracy, and age. She wants to be confident that any difference in final data-entry speed is actually caused by the software interface rather than pre-existing differences among the clerks. Which of the following procedures best applies the concept of random assignment to ensure group comparability?

A team of psychologists conducts an experiment with 200 participants to test a new reading intervention. They use a random number generator to assign participants to either the treatment group or the control group. After the study, a critic claims the results are invalid because the researchers did not measure and explicitly balance the groups based on participants' prior reading levels before beginning the intervention. Based on the role of random assignment, which of the following is the best analysis of the critic's claim?

A research committee evaluating a proposed experiment suggests abandoning random assignment in favor of manually sorting participants to ensure both groups have identical baseline anxiety scores. Adopting this suggestion would be a methodological improvement because manual matching guarantees that all other unmeasured participant characteristics will also be evenly distributed across the groups.