120 seedlings were picked at random from a nursery. Half of them… Step by Step

Step by step

The question

Question: 120 seedlings were picked at random from a nursery.

Half of them were picked at random to get extra light.

The extra light group grew taller.

What can the study say?

There are four choices.

First we solve it, the normal way.

Then we pick.

1 Two questions

To judge a study, we ask two questions.

How were the seedlings picked?

And how were the groups made?

2 How they were picked

The story says the seedlings were chosen at random from the nursery.

So: yes.

At random means all the seedlings in the nursery had the same chance to be picked.

So the group is a fair small copy of the whole nursery.

random: all seedlings in the nursery had the same chance

So what we find can be used for the whole nursery: true for all seedlings in the nursery.

3 How the groups were made

The seedlings were chosen at random to get extra light.

So: yes.

Chance made the groups, not the seedlings.

So the two groups are alike, except for the extra light.

random groups: alike, except for the extra light

So the difference comes from the extra light: the extra light causes it.

Careful!

Only for the nursery they were picked from.

Not for all seedlings everywhere.

Careful: not true for all seedlings everywhere ✗

4 What the study can say

So the study can say: for all seedlings in the nursery, the extra light causes this.

causes it, for the whole nursery

5 Check

Let's check with the story.

The first sentence says: chosen at random from the nursery.

That matches.

"chosen at random from" → all the nursery

The second sentence says: chosen at random to get extra light.

That matches too.

So the answer is right!

"chosen at random to" → causes

6 Pick the choice

Our answer is causes it, for the whole nursery.

That is choice D.

D) causes it, for the whole nursery

7 Why not the others

A is linked, for people like these.

Wrong: random groups and a random sample: it causes it, for the whole nursery.

A) linked, for people like these ✗ random groups and a random sample: it causes it, for the whole nursery

B is linked in the nursery, maybe not caused.

Wrong: the groups were made at random, so the light causes it.

B) linked in the nursery, maybe not caused ✗ the groups were made at random, so the light causes it

C is causes it, for people like these.

Wrong: the seedlings were picked at random, so it holds for the whole nursery.

C) causes it, for people like these ✗ the seedlings were picked at random, so it holds for the whole nursery

Let's look back.

First: two questions.

Then: how they were picked.

Then: how the groups were made.

Then: what the study can say.

Then: check.

Then: pick the choice.

Last: why not the others.

Answer