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Getting a conclusion

Think Like a Scientist

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Sometimes, the examiner may give your child various setups.

Your child has to choose the correct setups so that he or she can give the conclusion.

Try out this question

Four cages containing the same type of fish, W, X, Y and Z were placed in different parts of a river as shown in the diagram.

The arrow shows the direction of river flow.

After one week, the results from the study for water pollution were obtained and recorded in the table below.

Based on the results obtained from the experiment above, which of the following conclusions is correct?

1) The water in deeper parts of the river is less polluted.

2) The water downstream is as polluted as water upstream

3) The water upstream is more polluted than the water downstream.

4) The water upstream is not polluted at all.

What is the analysis?

This question wants you to identify the correct conclusion.

What must you do to derive a conclusion?


you need to make sure the experiment is fair!


In any science experiment, you need to ensure that the experiment is conducted fairly so that you can write out your conclusion.

What is a fair experiment?

There is only one changing variable.

What is the correct answer?

Let us analyse the table of results.

The first clue is all cages show a decrease in the population size of fishes.

That means the water in all parts of the river is polluted.

Hence, we cannot choose option 4.


To find out if option 1 is correct, we compare Cages W and X.

(The number of fish at the beginning of the experiment is kept the same.)

Why do we have to keep the number of fishes the same?

If a cage has more fishes than the other, more fishes might die due to the lack of food.

This will affect the experimental results.

We need to make sure that the fishes die due to pollution and not from the lack of food.

Based on the results (comparing Cages W and X),

We can see that 40 – 15 = 25 fishes die in Cage W at the shallower depth of 3m.

We can see that 40 – 20 = 20 fishes die in Cage X at the deeper depth of 5m.

Conclusion: The water in deeper parts of the river is less polluted.

Next, we compare Cages Y and Z.

(The number of fishes at the beginning of the experiment is kept the same.)

We can see that 25 – 15 = 10 fishes die in Cage Y at the deeper depth of 5m.

We can see that 25 – 11 = 14 fishes die in Cage Z at the shallower depth of 3m.

Conclusion: The water in deeper parts of the river is less polluted.


The correct answer is Option 1.


What about options 2 and 3?

Option 2:

The conclusion mentions “Water downstream is as polluted as water upstream.”

What option 2 means is that the number of fishes dies at each cage must be the same.

Unfortunately, that is not true. The results show different number of fishes have died at each cage.

Moreover, the initial number of fishes at each cage is different.

We cannot choose option 2.

Option 3:

The conclusion mentions “The water upstream is more polluted than the water downstream.”

What option 3 means is that the number of fishes that die at the cage upstream is more than the number of fishes that have died downstream.

In other words, we should see more and more fishes die in each cage from Cage W to Cage X to Cage Y and to Cage Z.

However, the number of fishes that have died does not increase at each cage.

The reason is the initial number of fishes in each cage is different!

Hence we cannot choose option 3 as the conclusion.

What are the learning pointers?

1) Be a good detective!

Spot the correct clues in the question.

Example: Conclusion, depth of water, upstream and downstream

2) Think like a scientist.

Relate the clues to the correct science knowledge.

“Oh, I know that to write out my conclusion, I need to ensure the experiment is fair

“Ah I see. The depth of water is a changing variable

These are needed to help you pick and choose the correct cages.

3) Be accurate.

We cannot choose option 2 and 3 as the initial number of fishes are not the same.


Want to learn more?

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