Independent variable is the variable for which values are changed or selected by the investigator. Be careful not to confuse the different types of variable in an experiment. A useful way to remember the difference is that your results depend on what you do so they are your dependent variable. To make sure it is a fair test you need to control other things that could affect the result so they are your controlled variables. Which only leaves the thing which you deliberately change, that is your independent variable.
Just ignore the reference to OCR, the explanation of what is meant by an independent variable is just as relevant to AQA.