In the next activity, you will need to identify the basic principles of type of experimental design called the completely randomized block design. You’ll also need to and describe the role blocking plays and how to use a completely randomized block design to achieve a research objective.

Randomized Design

In previous activities, you learned that the design of an experiment can have a major impact on its results. If an experiment is not designed well, its results may be meaningless and misleading.

Good Experimental Design

In Questions 1 – 3 below, you’ll consider a large and important study as your deepen your understanding of the key components of an experiment, and pick up some new terms. Recall the key components of experimental design you learned about in section [2C]: factor of interest, response factor, nuisance factors, treatments, and random assignment. Can you describe what role each of these plays in an experiment?

Recall that researchers conduct experiments by assigning subjects (sometimes called experimental units) to certain experimental conditions or treatments (also called levels of the factor of interest) and then observing outcomes on the response variable. One experimental condition is the treatment of interest — this is what the researcher changes or manipulates to see if it causes a change in the response variable. You learned that the other group is called the control group. The control group does not receive the treatment of interest and is usually kept under conditions considered typical or common.  The control group is essential to the experiment. It shows that, all else being equal, the factor of interest is either making a change in the response variable or it isn’t. There are some particular techniques that researchers use to make sure all the treatment groups are indeed otherwise equivalent. 

• A placebo is a type of control treatment. A placebo is a harmless version of the treatment that does not contain any active ingredients (e.g., a sugar pill). It will typically look, taste, and smell like the treatment of interest or mimic it so that the two treatments appear identical to the subjects; this way the subjects don’t know which treatment they are receiving. A phenomenon called the placebo effect can sometimes, though, introduce ambiguity in the assessment of any changes occurring in the response variable. You’ll read more about the use of a placebo and the placebo effect in Question 1 below.
• Researchers will use a technique called blinding, in which the subjects do not know which treatment they received. A double-blind study is one in which neither the subject nor the people collecting data on the subjects know about the treatment assignments. Other blinding levels exist as well, especially in medical drug trials, including ones in which only the top level directors of a study have information about which treatment the patient receives. The patients, data collectors, and even the patients’ doctors and the people analyzing the data don’t know who received the treatment and who got the placebo.

Look for the terms placebo, placebo effect, blind, and double-blind in the questions and examples below.

[A choose-your-study example could be placed here to mimic Question 1 – 3]

Now it’s your turn to explore a large and influential experiment as you answer Questions 1 – 3. Keep your eye out for the key components and terms you’ll seen so far.

Now, compile a list of the key components that you identified in Question 1. Use them to answer Question 2.

In Question 3, you’ll fill in the blanks to describe how many participants were assigned into each group and what was compared at the end of the experiment so that conclusions might be drawn.

The Salk vaccine experiment was one of the largest health experiments ever conducted. It was an experiment because subjects were given treatments, but it’s not always possible to conduct experiments. Ethical issues, cost, time, and other considerations sometimes prohibit the use of an experiment. For example, it would be unethical to conduct an experiment on the effects of texting while driving with two treatment groups: those who text while driving and those who don’t. We would need to request half the subjects to engage in a behavior that we already suspect might cause injury or fatality. In this case, an observational study would be more appropriate, in which previously recorded vehicle collision data is observed to understand the effects of texting while driving.

The Salk Vaccine experiment used a completely randomized design because the experimental units (subjects) were randomly assigned to one of two (or more) treatments. Another type of experimental design called block design, or blocking, can be added to a completely randomized design to strengthen the results. Let’s explore this type of design now.

Completely Randomized Block Design

In a completely randomized block design, the experimental units (participants) are divided into homogenous (similar) groups called blocks. The blocks help to minimize the potential effects of a nuisance factor–a factor the researcher is not interested it, but one that can cause variation in the response factor. Some well-known nuisance factors that are often controlled for include environmental factors like temperature, light, time of day, or physiological factors in living subjects like weight, sex, or age.

To use a block design, the researcher will chose a factor to control, such as age. All subjects exhibiting a similar characteristic of this factor (say they are close in age for example, or are all of a particular gender) are grouped together in similar sized blocks. Then they are randomly assigned to the treatment groups. This will ensure that any variability due to the nuisance factor is evenly distributed across the treatments, and any variability in the response variable attributed to the nuisance factor is isolated and minimized.

To summarize, in a completely randomized block design, the experimental units are first grouped into homogenous (similar) blocks, then randomly assigned to treatments within each block.

Now you try answering Question 4, about nuisance factors.

Now you try it. Assess your understanding of blocking by answering Question 5.

Summary

In this What to Know page, you learned some new terminology and explored a new experiment type: completely randomized block design. You also learned about the placebo effect and nuisance factors.  Let’s summarize where in the text these new terms and design appeared.

• In Question 1, you identified the key components of an experiment.
• In Question 2, you determined whether or not an experiment has been designed well.
• In Question 3, you used a diagram to explain the design of a given experiment.
• In Question 4, you determined factors other than the explanatory variable that could have an effect on the response variable of a given experiment (nuisance factors)
• In Question 5, you identified blocks in a specified experiment.

If you feel comfortable with these ideas, it’s time to move on to Forming Connections.