Step by step
The question
Question: 140 students were chosen at random from a college.
They chose for themselves whether to eat breakfast every day.
The breakfast group had more energy.
What can the study say?
1 Two questions
To judge a study, we ask two questions.
How were the students picked?
And how were the groups made?
2 How they were picked
The story says the students were chosen at random from the college.
So: yes.
At random means all the students in the college had the same chance to be picked.
So the group is a fair small copy of the whole college.
So what we find can be used for the whole college: true for all students in the college.
3 How the groups were made
The students chose for themselves.
So: no, not at random.
People who choose to eat breakfast every day may be different in other ways too.
Careful!
Doing better does not prove the breakfast caused it.
Something else may make the difference.
So the breakfast and the better result go together.
But maybe the breakfast did not cause it.
4 What the study can say
So the study can say: in the college, the breakfast and the result go together, but maybe the breakfast is not the cause.
5 Check
Let's check with the story.
The first sentence says: chosen at random from the college.
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.