Independent, dependent and controlled variables
The independent variable is the factor deliberately changed, such as temperature in an enzyme investigation. The dependent variable is the outcome measured, such as time for starch to disappear. Controlled variables are kept constant so any change in the result can be attributed to the independent variable.
Name the quantity and, where useful, how it is measured. “Amount of enzyme” is weaker than “volume and concentration of amylase solution”.
A control experiment is not a controlled variable
A control experiment is a separate comparison set-up in which the factor being investigated is absent or inactive. For example, a boiled-enzyme tube shows whether an observed change requires an active enzyme. Controlled variables, by contrast, are conditions kept the same across all test set-ups.
How to write a fair-test answer
Use the pattern: “Keep [named quantity] the same by [method].” Examples include using the same volume measured with a pipette, keeping pH constant with a buffer, or using cylinders cut to the same length and diameter.
Do not list the independent variable among the controls. Do not say “repeat to make it fair”; repeats improve reliability, while controlled variables make the comparison fair.