Ok fasten your seatbelt… things are going to get a little bumpy….there are more big words ahead

Variables and Fair Testing. Odd results, Accuracy and Precision

Fair Tests – have you heard of this before?  And no it’s not when your maths teacher gives you a revision list for a test, and then puts a whole load of stuff in which you haven’t learnt about yet.  Although that isn’t a fair test!

A fair test in Science is when you only change 1 thing in your experiment.

Variables make your investigation fair, they make your results comparable to other peoples’, they make them valid and they also help you think of all the things that you are going to do in your practical.


For example if you are testing the rate at which sugar dissolves – you can’t use the time it takes 2 teaspoons of sugar in hot water in one beaker and compare that to the time it takes 3 teaspoons of sugar to dissolve in cold water.


There are 3 types of variables, they have several different names but they all mean the same thing!


What you are changing is called the INDEPENDENT variable or the input variable.  This is what you are actually testing – this is the point of the investigation.



What you are measuring is called the DEPENDENT variable or the output variable.  This is stuff like the time something takes, or the temperature it gets to.


All the stuff you are keeping the same are called the CONTROLS.


You must remember that you can only change 1 thing and measure 1 thing.  You also need lots of controls.

If your aim is to see how temperature affects the rate of dissolving then you must use the same amount of sugar each time. You must use the same amount of water each time.  You will be measuring the time taken.

So in this example:

Variables you will be keeping the same (controlled variables):  volume of water and mass of sugar, type of sugar, grain size of the sugar

Variable you will be changing (independent variable):  temperature of water

Variable you are measuring (dependent variable):  time taken for dissolving to happen

Validity:  This means that you can hang your hat on your experiment – you can compare your results, make predictions with the confidence of knowing that you have done a good job.


This is the evaluation of the scientific merit of an experiment. There are 3 aspects to ensuring the validity of an investigation:

  • Fair testing

  • Making sure the set of data that you collect answers your investigative question, or proves what you set out to prove

  • Range of data collected – you need to think about the intervals in your readings – have you given enough time for something to happen?



I am sure your parents measured your height on a wall when you were little – and probably did it about once a year.There was probably quite a difference in your height each year – now imagine if it was done every day – would this be a suitable amount of time in between each measurement? Would there be a significant difference in your height?Probably not… unless you have been put on a stretching machine….


If an investigation is not a fair test, it is unlikely to be valid. Therefore it is crucial to ensure your design is fair if you want the results to be valid.

In doing an investigation on how the amount of sunlight affects the growth of seedlings, would it be  valid to expose the seedlings to 2 minutes, 3 minutes, 4 minutes, 5 minutes and 6 minutes of sunlight per day (a range of 2 to 6 minutes)?


 Would it be better to use daily intervals?

Are you going to notice any signigicant change if you are checking on your seedlings every few minutes??  Probably not, so in this instance measuring in daily intervals would make your measurements more valid.

The validity of an investigation is increased by the number of readings taken within the range. It is advisable to have at least 5 readings.

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All images sourced from wikicommons.org unless otherwise stated