Key Stage 3 science · Working scientifically

KS3 science investigations: variables, planning and fair tests

In Key Stage 3 science, pupils move from following instructions to designing their own investigations. They turn an observation into a testable question, make a prediction using scientific knowledge, decide what to change, measure and keep the same, carry out the work safely, and then judge how reliable their method was and how to improve it. These skills are assessed throughout science lessons and carry straight on into GCSE.

Planning science investigations, explained

An investigation starts with a question that came from noticing something: why does the ice on the path melt faster where salt has been put down, or why are there more daisies on the school field near the path? Pupils use what they already know to turn this into a line of enquiry, and then make a prediction. A prediction is not a guess. It states what they expect to happen and gives a reason from scientific knowledge, such as “I predict a longer pendulum will take longer to swing, because the bob has further to travel”.

Next comes the choice of method. The independent variable is the one the pupil deliberately changes. The dependent variable is the one they measure to see the effect. Control variables are everything else that must be kept the same so that the test is fair. Pupils also learn that the “fair test” is only one type of enquiry. Others include looking for patterns (for example, do taller pupils have longer arm spans?), classifying and identifying, observing change over time, and researching using secondary sources. Part of the skill is choosing the type that suits the question.

Pupils choose suitable apparatus and use it properly, in the laboratory and outdoors during fieldwork, with health and safety in mind. They record observations and measurements in a table prepared in advance, using a sensible number of readings and the right units. Fieldwork often involves sampling: you cannot count every daisy on a field, so you count in a number of small squares called quadrats placed at random, then use the results to estimate the whole field. Random placement avoids bias, and more quadrats give a more trustworthy estimate.

Finally pupils evaluate. They ask whether the method gave reliable results, which readings looked odd and why, and what they would change: more repeats, a more precise instrument, a better way of keeping a control variable the same. Being able to say “this part of my method was weak, and here is a specific improvement” is worth more than claiming everything went perfectly.

Words your child will hear

Hypothesis and prediction: A hypothesis is a possible explanation that can be tested. A prediction says what you expect to happen, with a scientific reason.
Independent variable: The thing you deliberately change in an investigation.
Dependent variable: The thing you measure to see what effect the change has.
Control variable: Anything else that could affect the result, which you keep the same.
Repeat readings: Taking the same measurement again, often to work out a mean, so a one-off slip does not mislead you.
Sample: A small, carefully chosen part of something large that you measure to estimate the whole.
Reliable: Giving results you can trust, because repeating the method gives much the same answer.

What the National Curriculum says

In Key Stage 3, pupils should be taught to:

Ask questions and develop a line of enquiry based on observations of the real world, alongside prior knowledge and experience
Make predictions using scientific knowledge and understanding
Select, plan and carry out the most appropriate types of scientific enquiries to test predictions, including identifying independent, dependent and control variables, where appropriate
Use appropriate techniques, apparatus, and materials during fieldwork and laboratory work, paying attention to health and safety
Make and record observations and measurements using a range of methods for different investigations; and evaluate the reliability of methods and suggest possible improvements
Apply sampling techniques

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: planning a pendulum investigation

Plan an investigation into whether the length of a pendulum affects the time it takes to swing.

Question and prediction. “Does the length of a pendulum affect the time for one swing?” Prediction: a longer pendulum will take longer for each swing, because the bob has a longer path to travel.
Variables. Independent: the length of string, from the pivot to the middle of the bob (20, 40, 60, 80 and 100 cm). Dependent: the time for one complete swing, there and back. Control: the mass of the bob, the starting angle (a small one, about 20 degrees), the same stopwatch, and the same person timing.
Method. Time 10 complete swings rather than one, and divide by 10, because one swing is too quick to time accurately. Do this three times for each length, and draw up the results table before starting.
```
Length   10 swings (s)    1 swing
(cm)     1st  2nd  3rd    mean (s)
 20
 40
 60
 80
100
```
Safety. Clamp the stand to the bench or weigh it down with a heavy object, keep the string tied firmly, stand clear of the swinging bob, and keep bags and stools out of the way.
Evaluation. Reaction time when starting and stopping the stopwatch is the biggest weakness. Timing 10 swings reduces its effect. Starting and stopping the timer as the bob passes a fixed marker at the bottom of the swing helps, and filming the swings in slow motion or using a light gate would improve it further. If one reading is far from the other two, repeat it.

A well planned investigation names the question, a reasoned prediction, the independent, dependent and control variables, how many readings are taken, the safety precautions and an honest evaluation of what could be improved.

Common mistakes and how to help

Thinking every investigation is a fair test

Early practical work often uses fair tests, so pupils assume that is what “an experiment” always means.

What helps: Ask what kind of question it is. “Which leaves are most common here?” needs sampling. “What do ice cubes do over an hour?” needs observation over time.

Mixing up the independent and dependent variables

Both words are long and similar, and pupils tend to describe the thing they are most interested in as the one they “change”.

What helps: Use the sentence “I change the ... and measure the ...”. The first blank is independent, the second is dependent. Check the graph: the independent goes on the horizontal axis.

Treating a prediction as a guess

In everyday language “predict” and “guess” mean the same, so pupils write “I think it will go up” with no reason.

What helps: Ask “because...?” until there is a piece of science in the answer. A prediction that turns out wrong is still a good one if the reasoning was sensible.

Believing that more repeats make a poor method good

Pupils learn that repeating makes results more reliable and conclude it is the cure for any problem.

What helps: Repeating helps with random slips. It cannot fix a flaw built into the method, such as a poorly controlled variable or an instrument that reads too high.

How to help at home

Turn “I wonder” into a question. When your child wonders why something happens, help them rewrite it so it can be tested: “Does the kind of paper change how far a paper aeroplane flies?” Then ask what they would change, measure and keep the same.
Run a safe kitchen test. Put identical ice cubes into cups made of different materials, such as a mug, a glass and a paper cup, and time how long each takes to melt. Or measure how far a toy car rolls when a ramp is set at different heights. Ask your child to name the three variables first and record the results in a table they have drawn up in advance.
Sample the garden or park. Mark out a small square with string, or use a hoop, and place it at random by throwing a beanbag over your shoulder. Count the daisy plants inside. Do ten throws and work out the mean per square. Then estimate how many of your squares would fit on the whole lawn and multiply to estimate the total. Stay away from roads and ponds.
Evaluate after, not just before. When a home experiment is finished, ask what was hard to keep the same, which reading seemed odd and what they would do differently next time. A specific answer is the goal.

Builds on

Years 5 and 6: Working scientifically

Leads on to

Key Stage 4: The development of scientific thinking
Key Stage 4: Experimental skills and strategies
Key Stage 4: Analysis and evaluation
Key Stage 4: Units, symbols and nomenclature