In the next activity, you will need to identify blinding, placebos, blocks, and nuisance factors as components of good and completely randomized block designs. A randomized block design is a type of experimental design that places similar participants into groups called blocks. The individuals in each block are then randomly assigned to treatments. We’ll learn that, in addition to control and experimental groups, a placebo can be assigned as a treatment.

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. Let’s review the key components of an experiment, along with the role that each component plays in its design. This will help you to better understand how the design of experimental studies can affect how we infer conclusions from analyses of data.

Recall that researchers conduct experiments by assigning subjects to certain experimental conditions or treatments and then observing outcomes on the response variable. A placebo is one of the treatments. It 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.

In Questions 1 – 3, you’ll consider a famously important study as your refresh the key components of experimental design.

The Salk Vaccine Trials

In the 1950’s, a large-scale experiment was designed to test the effectiveness of the Salk vaccine in preventing polio, which had killed or paralyzed thousands of children. Children were assigned to either the experimental or control group through a process of random assignment. Suppose that 200,745 children were given a treatment consisting of Salk vaccine injections, while a control group of 201,229 children were injected with a placebo that contained no drug. The children being injected did not know whether they were getting the Salk vaccine or the placebo. Among the children given the Salk vaccine, 33 later developed paralytic polio, but among the children given the placebo, 115 later developed paralytic polio.

Before you begin Question 1, assess your understanding of each of the following terms. You saw these in 2C: Experimental Design. Refer to your notes from that section as necessary.

• Factor of Interest
• Response Variable
• Treatments
• Random Assignment

You’ll also see some new terminology, including the following. Watch for these and make a note of their definitions, which will be given as the terms appear in Question 1

• Experimental Units (participants or subjects)
• Placebo, and the Placebo Effect
• Blinding
• Double-Blind

The Salk vaccine experiment was one of the largest health experiments ever conducted. It was an experiment because subjects were given treatments, but ethical issues, cost, time, and other considerations sometimes prohibit the use of an experiment. For example, it would be unethical to conduct a texting-while-driving experiment in which we ask subjects to text while driving because of the risk of injury or fatality. An observational study would be more appropriate in which previously recorded vehicle collision data could be observed to understand the effects of texting while driving.

The experiment described in Question 1 used a completely randomized design because the experimental units (subjects) were randomly assigned to one of the treatments. Another type of experimental design called block design (blocking) can be added to a completely randomized design to strengthen the results. Let’s explore this type of design now.

[above 2 paragraphs rewritten from the original]

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 that can cause variation in the response factor but also that the researcher is not interested it. Nuisance factors that are well known and 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, 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.

See the video below for a demonstration of how blocking works, then answer Question 4.

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Recall that in an experiment we want to know what effects the factor of interest has on the response variable. Sometimes a nuisance factor may also affect the response factor, even though we are not interested in this factor (e.g., the specific operator who prepared the treatment, the time of day the experiment was run, the room temperature). All experiments have nuisance factors, so the experimenter will typically need to spend some time deciding which nuisance factors are important enough to keep track of or control.

Sometimes the nuisance factors can be directly controlled in the experiment using a completely randomized block design. This design is used when the experimental units are divided into homogeneous groups called blocks. A block is a group of subjects that are similar, but blocks differ in ways that might affect the outcome of the experiment. For example, if your nuisance factor is known and controllable, it can be added to your experimental design. We use the term blocking to describe the grouping together of homogeneous (similar) experimental units, followed by the random assignment of the experimental units within each group to a treatment.

In a completely randomized block design, we do not wish to determine whether the differences between blocks result in any difference in the value of the response variable. Our goal is to remove any variability in the response variable that may be attributable to the block. Therefore, the advantage of this design is that blocking will help to minimize the effects of nuisance factors.

The basic principles of the completely randomized block design are blocking and randomization. The general rule is to “block what you can and randomize what you cannot.” So, blocking is used to remove the effects of a few of the most important nuisance factors. Randomization is then used to create comparable groups to reduce contaminating effects from the removal of the nuisance factors.

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.