OCR A Level Biology:复习笔记1.1.2 Identification of Variables

Identification of Variables

Types of variables

  • In an experiment, a variable is any factor that could change or be changed
  • There are different types of variables within an experiment
    • The independent variable: the only variable that should be changed throughout an experiment
    • The controlled/confounding variables: any other variables that may affect the results of the experiment that need to be controlled or monitored
    • The dependent variable: the variable that is measured to determine the outcome of an experiment (the results)


  • It is essential that any variable that may affect the outcome of an experiment is controlled in order for the results to be valid
  • Preliminary research and preliminary studies can be used to identify variables within an experiment and to determine ways of controlling these variables effectively
  • The science surrounding the issue/problem being investigated is likely to contain information about different factors or variables that may exist

Example of the science surrounding enzyme rate experiments

  • Enzyme rate experiments are experiments that are carried out to determine the effect of changing a particular factor on the rate of a reaction that is catalysed by an enzyme
  • Factors that can be changed include:
    • Temperature
    • pH
    • Enzyme concentration
    • Substrate concentration


  • The key thing with enzyme rate experiments is to ensure that only one of these variables is changed during a particular experiment
    • This is known as the independent variable


  • All other variables must be controlled (they must stay the same)
    • These are known as the control variables


  • For example, if investigating the effect of temperature on the rate of reaction, the pH, enzyme concentration and substrate concentration must be exactly the same (kept constant) each time you run the experiment (at each different temperature you are investigating)
  • If these control variables are not kept constant, they could affect the results of the experiment
    • This would make the results unreliable