Step by step
The question
Question: 300 teens from a town volunteered for the study.
They chose for themselves whether to read before bed.
The reading group slept longer.
What can the study say?
1 Two questions
To judge a study, we ask two questions.
How were the teens picked?
And how were the groups made?
2 How they were picked
The story says the teens 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 town.
So what we find is true only for people like these, not for the whole town.
3 How the groups were made
The teens chose for themselves.
So: no, not at random.
People who choose to read before bed may be different in other ways too.
So the reading and the better result go together.
But maybe the reading did not cause it.
4 What the study can say
So the study can say: the reading and the result go together, only for people like these.
Maybe the reading 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.