110 swimmers from a club volunteered for the study. Half of them… Step by Step

Step by step

The question

Question: 110 swimmers from a club volunteered for the study.

Half of them were chosen at random to train in the morning.

The morning training group swam faster.

What can the study say?

1 Two questions

To judge a study, we ask two questions.

How were the swimmers picked?

And how were the groups made?

2 How they were picked

The story says the swimmers volunteered.

They chose to join.

So: no, not at random.

People who join by themselves can be different from the others.

volunteers: only people who wanted to

Careful!

A big group is not enough.

Volunteers may be different from the rest of the club.

Careful: not 110 swimmers → true for everyone ✗

So what we find is true only for people like these, not for the whole club.

3 How the groups were made

The swimmers were chosen at random to train in the morning.

So: yes.

Chance made the groups, not the swimmers.

So the two groups are alike, except for the morning training.

random groups: alike, except for the morning training

So the difference comes from the morning training: the morning training causes it.

4 What the study can say

So the study can say: the morning training causes this, but only for swimmers like these.

= causes it, for people like these

5 Check

Let's check with the story.

The first sentence says: volunteered.

That matches.

"volunteered" → only people like these

The second sentence says: chosen at random to train in the morning.

That matches too.

So the answer is right!

"chosen at random to" → causes

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.

Answer