Step by step
The question
Question: 250 drivers from a city volunteered for the study.
Half of them chose to listen to music while driving.
The music group felt calmer.
What can the study say?
1 Two questions
To judge a study, we ask two questions.
How were the drivers picked?
And how were the groups made?
2 How they were picked
The story says the drivers volunteered.
They chose to join.
So: no, not at random.
People who join by themselves can be different from the others.
Careful!
A big group is not enough.
Volunteers may be different from the rest of the city.
So what we find is true only for people like these, not for the whole city.
3 How the groups were made
The drivers chose for themselves.
So: no, not at random.
People who choose to listen to music while driving may be different in other ways too.
So the music and the better result go together.
But maybe the music did not cause it.
4 What the study can say
So the study can say: the music and the result go together, only for people like these.
Maybe the music is not the cause.
5 Check
Let's check with the story.
The first sentence says: volunteered.
That matches.
The second sentence says: they chose.
That matches too.
So the answer is right!
Let's look back.
First: two questions.
Then: how they were picked.
Then: how the groups were made.
Then: what the study can say.
Last: check.