The independent (manipulated) variable is what you change on purpose. The dependent (responding) variable is what you measure to see the effect. Everything else you keep the same is a controlled (fixed) variable.
One sentence that settles it
Most aims can be rewritten as: “I change ___ to see what happens to ___.” The first blank is independent and the second is dependent.
Try it on an original aim: “To study how the length of a pendulum affects its time for 10 swings.” I change the length of the string to see what happens to the time for 10 swings. So length is independent and time is dependent.
Finding the controlled variables
Ask what else could change the result. For the pendulum, the list includes:
- mass of the bob, because a heavier bob might swing differently
- the starting angle
- the same stopwatch and the same person timing
- the point where the string is held
Each one is kept the same so that length is the only explanation for any change in time. Give a short reason for each controlled variable, because that scores better than a bare list.
A common mistake
Students often pick the variable that sounds most important instead of the one that was changed. In “the effect of fertiliser amount on plant height”, some say plant height is the independent variable because the plant is the main thing in the experiment.
Ask what the experimenter does. They add different amounts of fertiliser, so fertiliser amount is independent. They measure height with a ruler, so height is dependent.
What if the aim is not stated?
Some questions show only a table. Look for the column with neat steps, like 10 cm, 20 cm, 30 cm. The experimenter chose those values, so it is the manipulated variable. The column with uneven results is the one that was measured.
Try it yourself
A student tests whether the temperature of water affects how long 5 g of salt takes to dissolve. Name the independent, dependent and two controlled variables.
Answer
Independent: temperature of the water. Dependent: time taken for the salt to dissolve.
Controlled variables could be the mass of salt (5 g), the volume of water, the size of the salt grains or the amount of stirring. Any two that could change dissolving time are acceptable. The frame “I change temperature to see what happens to dissolving time” confirms the first two.
Next steps
The variables go straight onto a graph, so read reading a graph before drawing a conclusion. The Mathematics side is in choosing axes and scales. To practise with a teacher, try SPM Science tuition, and the hub lists the other skills.