Describe a completely randomized design for an experiment.
Describe a randomized block design and a matched pairs design for an experiment and explain the purpose of blocking in an experiment.
Activity: Does the Type of SAT Prep Matter?
This activity walks students through three different experimental designs, in increasing complexity.
Completely randomized design
Matched pairs design
Notice that the matched pairs design is really just a fancy version of a block design, where each block is of size 2 (two students paired with similar GPAs).
Which variable should be used for blocking? Why block?
Should we create blocks of Juniors and Seniors? Blocks of males and females? Blocks of high GPA and low GPA?
Which variable? Choose the one that is most strongly associated with the response variable. We think grade level is the variable that is most strongly associated with differences in SAT scores.
Why block? To reduce the variability in the response variable (SAT score). The variability of SAT scores should be less in each block then it would be for the whole group. This can help make it easier to determine if results are statistically significant (increase power). To dig deeper into this idea, consider doing the Dog Blocking Activity.
The thinking and reasoning here is very similar to making the choice of variable to use when doing a stratified random sample (like the choice of stratifying by row in the Justin Timberlake lesson). It is easy to look back at this lesson and make the connection (hindsight is 20/20).