Step 4: Assign your subjects to treatment groups
How you apply your experimental treatments to your test subjects is crucial for obtaining valid and reliable results.
First, you need to consider the study size: how many individuals will be included in the experiment? In general, the more subjects you include, the greater your experiment’s statistical power, which determines how much confidence you can have in your results.
Then you need to randomly assign your subjects to treatment groups. Each group receives a different level of the treatment (e.g. no phone use, low phone use, high phone use).
You should also include a control group, which receives no treatment. The control group tells us what would have happened to your test subjects without any experimental intervention.
When assigning your subjects to groups, there are two main choices you need to make:
A completely randomized design vs a randomized block design.
An independent measures design vs a repeated measures design.
An experiment can be completely randomized or randomized within blocks (aka strata):
In a completely randomized design, every subject is assigned to a treatment group at random.
In a randomized block design (aka stratified random design), subjects are first grouped according to a characteristic they share, and then randomly assigned to treatments within those groups.