Planning for a fair test

Improving Science Education 5-14

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An Investigative Approach to Science

Planning for a fair test
Why do we need fair testing?	Classroom activities	Glossary of terms

Why do we need fair testing?

It is only by carrying out a fair test that you can be sure that it is what you have changed (independent variable) that is affecting what you measured (dependent variable).

Notes:

It is easier to recognise that a test is fair than it is to plan and carry out a fair test.
You will need to encourage the children to make sure that all relevant aspects have been controlled (kept the same).
Most children need only say that they intend to keep certain things the same, but the most able children should be encouraged to discuss what value each control variable should have.

Some classroom activities to help to establish the concept of fair testing are shown overleaf. These involve the teacher showing the children, or describing to them a testing situation which is not fair. The children are then asked to suggest why it is not a fair test. So possible responses from the children are included in the green boxes.

Childrens
response

Classroom activities to help children learn about fair testing

1. Carry out a test in front of the children which you deliberately make unfair.

You pushed that one but you just let the other one go - that's not fair!

You measured one from the front and the others from the back - that's not fair!

You started one from higher up the ramp - that's not fair!

2. A test to see which surface is more bouncy. The teacher makes two different sized balls and drops them from different heights. She then asks: What is wrong with this test?

One ball is bigger than the other - that's not fair!

One ball was thrown - that's not fair!

One ball has to fall further than the other - that's not fair!

3. The teacher reads to the children these descriptions of tests carried out by others. She then asks: Which one is a fair test?

We wanted to find out how much water plants need to make them grow well. We took six germinating plants and gave them the different amounts of water each day. Three were kept on the window sill and three on top of the cupboard.

We wanted to find out how long it took parachutes made of different materials to fall to the ground. We made them the same size and dropped them from the same height. We then timed how long each took to fall.

4. The children are presented with the following list of variables, and asked to select the appropriate conditions for a fair test. The test is to identify the effect of changing the amount of water supplied on the growth of grass plants. The purpose of this activity is to show the children that there are many different possible choices for the control variables. The child needs to control all these by choosing one condition for each of these variables. The condition that they choose in each case is in a sense arbitrary.

My test - When I change the amount of water I give to the grass seeds, what will happen to the height the grass plant will be after three weeks?

Type of soil	sand	peat	garden soil
Amount of seed (spoons)	1	2	3
Depth of planting (cm below)	0 (surface)	0.5	2
Light (situation)	window sill	cupboard	table
Temperature (°C)	5	10	20

Glossary of terms
Investigation	Open-ended practical activity which the effect of one condition (independent variable) on another condition (dependent variable) is examined.
Independent Variable	Condition which we change during the investigation.
Dependent Variable	Condition which is affected by the independent variable.
Constant Variable	Condition which is kept the same throughout the investigation.
Fair Test	Experiment in which one condition (independent variable) affects another (dependent variable) by keeping all other conditions constant.
Prediction/Hypothesis	What we think will happen.
Result	What did happen.
Conclusion	Our explanation of this effect.
Brainstorming	Allowing uncritical expression of pupil ideas.