Learning focus
Develop transferable AO3 skills: plan, measure, record, process, interpret and evaluate biological investigations accurately and safely.
Independent variable
The independent variable is the factor deliberately changed by the investigator. It should be assigned a suitable range and intervals. If temperature is investigated, five or more values spread around the expected biological response normally reveal more than only two values.
Dependent variable
The dependent variable is the observation or measurement used to detect the response. It must be operationally defined. Enzyme activity could be measured as time for starch to disappear, product concentration per minute or gas volume per minute.
Control variables
Control variables are factors kept constant because they could influence the dependent variable. State both the variable and how it is controlled: “keep pH constant using the same buffer”, not merely “control pH”.
Confounding variables
A confounding variable changes with the independent variable and offers an alternative explanation. Moving a lamp changes both light intensity and temperature unless heat is controlled. Recognising confounding factors is central to validity.
Practical or data skill
For investigations of temperature on enzyme activity, light intensity on photosynthesis and exercise on pulse rate, identify one independent, one dependent and four control variables.
Examination tip
When asked how to control a variable, include the method, quantity or apparatus used.
Review questions and suggested answers
Question 1
What is the dependent variable?
Suggested answer
The factor observed or measured in response to the independent variable.
Question 2
Why control other variables?
Suggested answer
To ensure that any change in the dependent variable is attributable to the independent variable.
Question 3
Give one way to control pH.
Suggested answer
Use the same buffer solution at a specified pH.
