Fair tests and One variable at a time

Decoupling “fairness" from “one variable at a time"
Central to the vast majority of KS2-KS5 science investigations is the concept of “fair test". So ingrained is fairness, that pause and ask a leading question…."what do we need to do…." and your class will chorus “make it fair".
Fairness and all things fair testing is embedded into the KS3 and KS4 curricula.
  • Fairness is about identifying “the key variables that need to be controlled" and “how to change the independent variable whilst keeping other key variables the same".
  • Here at KS2, the focus is on keeping everything in an experiment as similar as possible between experimental runs and ONLY varying the independent variable - the often mentioned “one variable at a time approach"
  • Importantly, at KS2 the elements of fairness require that everything is controlled and only one thing is varied - this by definition is what makes it a “fair test" as KS2
When carrying out a fair test, control variables appropriately and identify any variables that cannot readily be controlled
  • Level 4: In a fair test enquiry, they recognise, with support, the variables to change and measure and those to be kept the same. They decide upon some basic success criteria.
  • Level 5: When planning a fair test, they identify key variables and distinguish between independent and dependent variables and those that they will keep the same.
  • Level 6: They make predictions using abstract scientific ideas. In a fair test enquiry, they plan how to control the variables that they need to keep the same and make decisions about the range and values of the independent variable.
  • Level 7: They identify key variables that may not be readily controlled explaining why this is the case.
  • EP: Plan to track changes in more than one dependent variable.
At KS3, the picture is somewhat more complex, but essentially the language of the levels talks about “controlling", meaning “keeping the same" EXCEPT the variable that you are going to change.
Again, this controlling everything by keeping things the same forms the definition of fair testing and fairness at KS3
  • How Science Works defines a fair test as: one in which only the independent variable affects the dependent variable. All other variables are controlled, keeping them constant if possible
  • An important, subtle and overlooked relaxation of the fairness rules here. At KS4 whilst the language talks about “the independent variable" - assuming that there is only one - the “keeping them constant if possible" is the welcome addition.
  • Control here at KS4 is not just about “keeping everything constant" - it’s about measured control and quantification of all the input variables.
  • However, as a science teacher it worries me that the level of sophistication between KS2 and KS4 with respect to fair testing and “fairness" does not seem to get deeper as learners progress.
But more worryingly, the confounding of “fair testing" with “one variable at a time" is a real and growing concern.
One variable at a time
Science in school is “one variable at a time" - we teach all our learners to keep everything as constant as possible, only change ONE input variable and measure output variable(s). By definition, we teach them, that if they deviate from this, the test is not “fair" and that we should question the results.
But that’s not how the vast majority of scientists and engineers work in the outside, wider world.
Many variables at once
The alternative approach is to change many variables at once, in a controlled manner - ( and control here means, ensuring that you KNOW what the values of these variables are - they don’t need to be fixed, invariant and hammered into stone). And here’s the rub - it’s still a fair test as long as you can be certain on your control.
Sure it’s more complex than one variable at a time, it might need the use of statistics to unpick the results - but, it’s what happens in the real world.
For example - germinating seeds can be effected by pre-soaking in water and pre-chilling - and importantly a combination of the two effects. For example pre-chilling at -12’C AND pre-soaking for 24 hours might produce a considerable improvement in germination rates.
This many variables at once can be summarised by a “Main Effects" plot - and this kind of investigation should not be beyond inclusion into the KS4 science curricula:

So, Science teachers - lets teach alternatives / additions to “one variable at a time". Exam boards, curriculum “authors" - let’s acknowledge that there are alternatives to one variable and that by changing more than one thing, its not automatically “un fair".

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