Fair Testing

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Learning Intentions

By the end of this topic, you should be able to:

Define what a fair test is and explain why it is important in science.
Identify independent, dependent, and control variables in an experiment.
Design a fair test by changing only one variable and keeping all others the same.
Explain how to make an experiment valid and reliable.
Evaluate the advantages and limitations of fair testing in different types of investigations.
Apply the concept of fair testing to real-world examples, such as comparing products or testing scientific ideas.

Glossary

Key terms and definitions.

control (in an experiment): The part of an experiment used for comparison, where no variable is changed.

control variables: The factors that must be kept constant throughout an experiment to ensure it is a fair test.

dependent variable: The variable that is measured or observed, to see how it responds to changes in the independent variable.

fair test: An experiment where only one variable is changed at a time, while all other variables are kept the same.

independent variable: The variable that is deliberately changed or tested to see its effect on the dependent variable.

reliable: When an experiment gives consistent results after being repeated under the same conditions.

valid: When the results of an experiment are trustworthy because the test was fair and measured what it was supposed to measure.

variable: Any factor that can change in an experiment, such as temperature, time, or material type.

What is fair testing?

Turning ideas and thoughts into scientific knowledge involves a process called investigating. Fair testing is the investigation approach that finds relationships between factors (variables). In a fair test, a single variable is changed, and another variable is measured. Meanwhile, other variables must be kept the same. This is so that the results can be used to make comparisons, and any differences are said to be the result of the changed / independent variable.

The variable that you are investigating and therefore change is called the independent variable. It will be altered over a range of values.
The variable being measured with respect to the independent variable is called the dependent variable.
Other variable that may affect the results being measured are called control variables. These variables must be controlled / kept constant so that we can say with confidence that any change in the dependent variable is caused by the independent variable. So, the key to fair testing is to keep as many of the things that can vary, the same for each test.

Example: Comparing washing powders

If a capful of one washing powder cleans a load of clothes, how do we know whether it is better or worse than other washing powders? You would want to carry out a fair test investigation that tests different powders with loads of clothes that were:

the same size
equally dirty
washed for the same length of time
washed at the same temperature
washed using the same amounts of powder

The only variable would be the product you are testing, which is the brand of the washing powder (e.g. Persil vs. Ecostore). This would determine which powder cleaned best for the same amount used. However, it would not take price into account. A product which is 'not as good' could be cheaper than a 'better' one. So, in another sense the cheaper product could still be best.

Experiments often include seeing the effect of a change. To do a fair test, you need to set up:

one experiment in normal conditions called a "control"
one or more experiments with one factor changed in each.

When testing washing powders, the control could be a wash with no washing powder to compare with the other results.

What are the advantages and limitations of fair testing?

Fair testing is most easily suited to technology investigations, for example, "Which paper towel can soak up the greatest volume of water?", and physical sciences, for example, "Will the reaction go faster if a more concentrated acid is used?"

Fair testing is particularly well-suited to investigations that record measurements.

This method will not work well where investigations:

need to be done in the field
are monitoring change over time
need to examine a whole system, not just isolated parts.

How do I plan fair test investigations?

Careful planning is required to ensure a fair test is carried out and the results are valid. The steps required when planning an investigation include:

Knowing what you are trying to find out

An aim gives the purpose of the investigation. It is very important that you know exactly what you are trying to find out. A hypothesis is a testable prediction or educated guess as to the outcome, and is based on what you already know.

Making sure it is a fair test

Knowing what all the variables are and controlling them (where possible) is important if observations and measurements are to be accurate and valid.

The independent variable is the one you change deliberately to see the effect it has, e.g. the force used to push the trolley.
The dependent variable is what you measure or observe and depends on the independent variable, e.g. the time the trolley takes to travel 1 metre (from this you can then calculate the trolley's speed). Any other variables that may affect the results need to be kept the same, e.g. the trolley used, the surface it is pushed on, the distance over which it is timed, how the trolley is pushed, what is used to time the journey.
A "control" is often used in biological investigations. The control differs from the experimental part of the investigation in only one way and that is the variable being tested. The control and experiment are compared to each other, e.g. starch testing leaves from a plant growing in the sun (control) and one grown in the dark (experiment).

Using the correct equipment

Make a list of everything needed to carry out the investigation. Know how to use all the equipment correctly and how to collect data accurately.

Have a clear easy-to-follow method

Write a method as a series of short steps that follow a logical order so another person could follow them and get the same results as you did. Underline key words to draw attention to important instructions. Include labelled diagrams if they help clarify what to do. You could do a trial run, to pick up any glitches and make corrections before the actual investigation.

How do I make my fair test investigation valid?

To try to ensure that the results of an experiment are valid, the experiment must be:

A fair test - during a fair test, all the things that can vary, such as the equipment used, or the temperature at which the experiment is carried out, are kept the same - except for the one thing that is being investigated by the experiment (independent variable).
Reliable - a reliable test or experiment always gives the same results. The experimental procedure should be repeated at least once to check that it produces the same results as the previous experiment.

Example

An experiment was carried out to decide if sea water has greater mass than fresh water. The procedure is shown in the diagram below.

Independent variable (variable changed)

Type of water used - sea water or fresh water.

Dependent variable (variable measured)

Mass of the water sample in grams.

Control variables (variables kept the same)

The volume of water used
The temperature of both types of water
The containers sed to weigh the water samples
The electronic balance used to weigh the water samples

This is a fair test because only one variable was changed, allowing a fair comparison of the mass of the two types of water.