This tutorial is going to teach you about a randomized block design. A randomized block design is a little bit different than other types of designs that we've studied, so this tutorial will focus on:
Suppose you are a researcher and you want to identify whether a new acid reflux drug is more effective than the one that's currently available. You gather 500 volunteers with acid reflux, put the number one on 250 cards, and the number two on another 250, and place all the cards in a hat. You mix them up and have people pull out numbers. People who received a "one" receive a new drug, those selected "two" received the old drug. The image below would be your original plan, starting with all these volunteers, men and women, and then you randomly assigned them to groups.
This method is the old design. The problem is, what if men and women respond differently to the drug?
The better design is called a Block design, so you try something different. First take your large group and break it into smaller subgroups of just men and just women.
The image above has 9 men and 14 women; you had a lot more in the old design, but now you’re going to run the experiments essentially in parallel: one experiment for men and one experiment for women.Now you’re going to take the men and randomly assign half of them to the treatment and half to the control. You’re going to take half the women and assign them to the treatment and assign them to the control, which looks like this:
Men receiving the treatment are in purple men receiving the control in in green. Women receiving the treatment in purple and women receiving the control in green.You might notice there are five men receiving treatment and only four receiving control. It’s not necessary to have exactly equally sized groups.
By doing a Block design rather than a completely randomized design, you can observe differences within the group that you might have missed had you done it with a large group.
Suppose the drug was more effective for women than for men. You would see that in this experiment here. You would see that the drug was effective for women. You would also see that it wasn't effective for men.
One minor disadvantage to running a Block design is that you do lose some of the replication that you would have had, had you run it in a large group. Sometimes you need to make your sample size a little bit bigger in order to overcome that. And it might be a little bit harder to draw legitimate conclusions with small groups.
In a randomized design, you saw how an experiment may miss an extra level of depth such as men and women reacting differently to a drug. The subjects or experimental units are grouped together by some similar characteristic that you think might affect the outcome. In this example, we use gender. When evaluating block design vs. randomized design, you saw that a Randomized Block Design, experiments run in parallel, resulting in two or more separate experiments. And then you can compare the treatments within each of those groups.
Source: this work is adapted from sophia author jonathan osters.
An experimental design where the subjects are separated into homogenous groups, called blocks, based on some variable we think may affect the outcome of the experiment. We then run the experiment separately within each block.