Overview

• In this in-class activity, students will add to their understanding of the key components necessary for a well-designed experiment by learning how to design an experiment using the completely randomized block design.
• Students will understand that a well designed experiment helps in determining whether the appropriate conclusion is made.
• This activity connects back to the key components of an experimental study, and prepares them to perform inference later in the course.
• S2, S3, C2, C3, C4, C5, C6, V1, V4, O3 ← Link to EBTP descriptions 

Prerequisite assumptions

Students should be able to do each of the following after completing the What to Know assignment.

• Identify the key components of an experiment.
• Determine whether or not an experiment has been designed well.
• Use a diagram to illustrate the design of a given experiment.
• Determine factors, other than the factor of interest, that could have an effect on the response variable of a given experiment (nuisance factors).
• Identify blocks in a specified experiment.

Intended goals for this activity

After completing this activity, students should understand the purpose of blocking and the importance of a well-designed experiment. They should be able to describe the role that blocking plays in an experiment, identify the basic principles of the completely randomized block design, and eescribe how to use a completely randomized block design for a given research objective.

Synchronous Delivery and Activity Flow

The sample activity delivery below assumes a face-to-face class meeting but can be adapted to a fully online or hybrid delivery by using break-out rooms for pairs and small groups.

Frame the activity (3 minutes)

• Question 1 — think-pair-share S2, C4, V1, V4, O3
  • Allow students one minute to think about the role that randomization plays in the experiment. Then allow each student to pair up with the person adjacent to them to share their thoughts.
  • Examine student responses by allowing select students to share their responses.
  • Help students understand that randomization allows the researcher to create comparable groups—that is, groups that have similar distributions of the other factors. In other words, the researcher is trying to ensure that the factors in one group are similar to the factors in the other group. For example, the health conditions of a subject could play a role in how subjects respond to a treatment (in this case, the Salk vaccine injection or the placebo injection). Random assignment helps to ensure that one group will not be much “healthier” than the other group.
• Transition to the in-class activity by briefly discussing the Objectives for the activity.

Activity Flow (13 – 15 minutes)

• Setting up the need for blocking by asking a few volunteers to share their answers to Question 5 from What to Know [2E]. S3, C4, C5
  • Consider asking, “What is the purpose of creating blocks?” Possible answer: The chief researcher believes level of education plays a role in the effectiveness of advertisement.
  • Reinforce that blocking will allow the researcher to control the effects of this possible difference.
  • Remind students that the basic principles of the completely randomized block design are blocking and randomization.
• Question 2 — working in groups V1, V4, O3, S4, C3, C4, C6
  • Guide students in completing the diagram illustrating the randomized block design. Consider asking the following questions as a guide:
    • “How do we determine which experimental units form the blocks? What is the purpose of blocking?”
      • Possible answer: By identifying any known differences that will have an effect on the response variable — the purpose of blocking is to minimize the effects of nuisance factors.
      • Remind students that the design is not limited to two blocks.
    • “How are the experimental units assigned to the experimental treatments?”
    • “Is the purpose of a completely randomized block design to determine whether differences between blocks result in any differences in the value of the response factor?”
      • Remind students that the goal is to remove any variability in the response variable that may be attributable to the block.
      • Results are compared within and between blocks.
  • The terms completely randomized block design, block, blocking, and blinding were all introduced in the preview assignment.
• Questions 3 – 6 — working in small groups S2, C4, V1, V4, O3, C6
  • Note that students may struggle with completing the template in Question 3. Provide support by walking around and answering questions.
    • Part A: “Is there a known difference that will have an effect on the response variable (the achievement test results)?” Remind students that we are interested in the effect of the teaching method rather than any differences between boys and girls, since we know that gender affects the response variable.
    • Part F: Remind students that achievement test results are compared within each block and across blocks.
• Wrap-up/transition (5 minutes)

• Review the key components of a well-designed experiment.
• Give students a few moments to answer the questions: “What are the basic principles of the completely randomized block design? What role does each principle play in the design?”
• Have students refer back to the Objectives for the activity and check the ones they recognize. Alternatively, they may check the objectives throughout the activity
• Summarize for students that they have learned in the previous several sections about different data collection methods, sampling bias, and basic and advanced experimental design.
• Assign the homework or Practice and any What to Know pages for the Forming Connections activities you plan to complete in the next class meeting. C2