90 people from a city signed up for the study. Half of them were… Step by Step

Step by step

The question

Question: 90 people from a city signed up for the study.

Half of them were chosen at random to take a vitamin.

The vitamin group had fewer colds.

What can the study say?

1 Two questions

To judge a study, we ask two questions.

How were the people picked?

And how were the groups made?

2 How they were picked

The story says the people signed up.

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 city.

Careful: not 90 people → true for everyone ✗

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

3 How the groups were made

The people were chosen at random to take a vitamin.

So: yes.

Chance made the groups, not the people.

So the two groups are alike, except for the vitamin.

random groups: alike, except for the vitamin

So the difference comes from the vitamin: the vitamin causes it.

4 What the study can say

So the study can say: the vitamin causes this, but only for people like these.

= causes it, for people like these

5 Check

Let's check with the story.

The first sentence says: signed up.

That matches.

"signed up" → only people like these

The second sentence says: chosen at random to take a vitamin.

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