(b) identification of variables that must be controlled, where appropriate

In any investigation, you deliberately change one variable and measure another. The variable you change is the independent variable. The variable you measure is the dependent variable. Everything else that could distort the result should be held constant. Those held-constant variables are controlled variables, and they include confounding factors. A confounding factor is something other than the independent variable that could affect the dependent variable. To identify controlled variables, ask what else could affect the dependent variable besides the independent variable. Those possible influences are the confounders. You control variables that could realistically affect the outcome, not every conceivable variable. The phrase 'where appropriate' means the control must be justified. For each one, explain how it could change the dependent variable if it were not held constant. Consider an investigation of enzyme activity against temperature. Temperature is the independent variable because it is deliberately changed. The rate of reaction is the dependent variable because it is measured. What else could affect rate? pH, substrate concentration and enzyme concentration are controlled variables because each can affect enzyme activity. If any of these changed between trials, the result could not be attributed to temperature alone. Now apply the same reasoning to a plant photosynthesis experiment. Suppose light intensity is deliberately changed, and the rate of oxygen production is measured. Carbon dioxide concentration and temperature are potential confounders because both can affect photosynthesis. They should be held constant so that any change in oxygen production can be attributed to light intensity. Notice that the independent and dependent variables are not the variables to control. A common mistake is naming the independent variable as the one to control. Remember, the independent variable is deliberately changed, not held constant. Another mistake is assuming every conceivable variable must be controlled. Control those that could realistically affect the dependent variable and justify the choice. Finally, do not ignore a confounder that clearly affects the outcome. Check each option against what could influence the measured result. Before choosing an answer, label each variable in the scenario as independent, dependent or controlled. Ask whether each option could affect the dependent variable; if it could, and it is not being changed, it needs controlling. Reject distractors that are the variable being measured or deliberately changed. Then briefly pause and try this: for a named control, explain how it could distort the dependent variable if it were not held constant.