Independent variable

The independent variable is the pH, established using labelled buffer solutions. The dependent measurement is the time taken for starch to disappear. Temperature must be controlled with a water bath or electric heater because temperature also affects enzyme activity.

The independent variable is sucrose concentration, while the dependent variable is the change in mass of the potato tissue.

A sound photosynthesis investigation changes only the intended independent variable and measures a suitable dependent variable. Starch-test investigations give a mainly qualitative result—starch present or absent—whereas collected oxygen volume gives quantitative data.

Light intensity is the independent variable and oxygen produced per unit time is the dependent variable. Keep the species and length of pondweed, carbon dioxide availability, measurement time and temperature constant. A lamp can warm the water, so temperature should be monitored; a water bath or heat shield can help to control it.