What is Randomized Block Design?

Instructor: Brekke Peterson Munks
Have you ever heard of a randomized block design? If not, you are in the right place! This lesson will explain what a randomized block design is and when one should use it. At the end of the lesson you can check your understanding with a short quiz.

What Is a Randomized Block Design?

Have you ever looked at a brick building and thought there is a perfect pattern to those bricks? Did you ever think of randomized block design? Of course you didn't because that term has nothing to do with architecture, but rather a way to statistically analyze a study and the data that comes out of it! Specifically, a randomized block design, or RBD, splits up experimental units into groups or blocks of equal size, and then assigns a treatment to each group randomly. This makes sure that the data is not affected by outside situations.

What Does a Randomized Block Design Look Like?

Let's pretend we are going to see how three different kinds of bean plants are affected by different water additions, ranging from no water to 100 milliliters of water. We will have 0, 5, 10, 25, 50, 75 and 100 milliliters of water we will add to the three plants.

To set this experiment up, we first must determine how many combinations are possible. We have three bean varieties we are testing with seven different amounts of water. We are controlling the bean variety and water addition - these are variables which we choose and are independent variables. We will monitor the growth of the plant varieties. This variable depends on the independent variable and is called the dependent variable. This experiment has 21 (3 * 7) treatment combinations of the plant variety and water amount factors. Each treatment will be repeated three times, producing a randomized block.

Example of a randomized block design, where three varieties of beans are tested to see how various water amounts affect them; replication is the blocking factor.

Basically an RBD looks like a way to organize each combination of variables to reduce any outside errors. Think of the RBD as the floors of an apartment building and each combination of factors is an apartment on one of the floors. The whole set of floors or blocks makes up the apartment building, which is like the whole experiment.

Why Use Randomized Block Design?

The use of randomized block design helps us to understand what factors or variables might cause a change in the experiment. In the bean example, the position of the plant was random so that would have no effect on the outcome of water on plant growth. There were three replicates of each water treatment and variety.

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