Concept
Ask what was randomized
A random draw into a group and a random allocation inside it solve different problems.
When it fits
- A study calls itself randomized and the word is doing most of the persuasion.
When to avoid it
- Randomization balances other influences in expectation, not perfectly in every small sample.
The idea
Separate selection from assignment. Selection concerns who enters the study; assignment concerns which condition participating units receive. Random assignment supports causal comparison when implemented and analyzed appropriately, but it does not automatically make a volunteer sample representative of everyone.
An example
Volunteers allocated randomly between two training formats remain volunteers, even when the comparison between formats is well designed.
Check your result
You can name the randomization step and the claim it supports.
Keep this limit in mind
- Randomization balances other influences in expectation, not perfectly in every small sample.
Connected ideas
Useful withHide condition labels from the person judging the outcome
Evidence and sources
Random assignment allocates participating units to conditions; it is distinct from randomly selecting units from a population.
Assignment balances differences in expectation, not perfectly in every realized sample, and does not establish population representativeness.
Introductory Statistics 2e, 1.4: Experimental Design and Ethics · Random assignment and experimental units