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TL;DR The independent variable (IV) is what you deliberately change; the dependent variable (DV) is what you measure in response; controlled variables (CVs) are everything else you keep constant so the test is fair. The number-one trap is naming the object rather than the measurable property -- writing "length of ruler" instead of "length of ruler overhanging the desk edge / cm" loses the mark every time. Listing three to four CVs with a one-sentence "how controlled" note consistently earns full marks on the planning question.
Every controlled experiment in Sec 1 and Sec 2 Science involves exactly three types of variable. You will be asked to identify them explicitly on the planning question, and each type has a precise definition.
Independent variable (IV): the one factor you deliberately change between trials. You choose the values, set them up, and vary only this one thing. There is always exactly one IV in a fair test.
Dependent variable (DV): the factor you measure as a result of changing the IV. The DV "depends on" what you did to the IV. You observe or record it; you do not control it.
Controlled variables (CVs): every other factor that could affect the DV but that you hold constant throughout all trials. Controlling them ensures that any change in the DV is caused by the IV and nothing else.
The mnemonic that works best: I change it, I measure it, I keep it the same. Point at each line as you read a question: what am I changing? what am I measuring? what else could affect the result that I must keep fixed?
The one factor deliberately changed between trials.
"What am I changing?"
Dependent variable (DV)
The factor measured to see the effect of the IV.
"What am I recording?"
Controlled variable (CV)
Any other factor kept constant so the test is fair.
"What else could affect my result if I let it change?"
2 | The ruler-bending-length trap
The ruler-overhanging-the-desk experiment is one of the most common Sec 1 practical setups. You clamp a ruler so that a measured length sticks out over the edge of a bench, press the free end down and release it, then time how long it vibrates or measure how far the tip bends. The investigation asks: how does the length of the overhang affect the vibration period (or bending)?
Here is how most students write the IV on their first attempt:
IV = length of ruler
That answer will not get the mark. The examiner's mark scheme requires the IV to be a measurable, quantitative property -- not the name of an object. "Ruler" is an object. It tells the reader nothing about what is actually changing between trials.
The correct answer:
IV = length of ruler overhanging the desk edge / cm
The difference matters for two reasons. First, specifying "overhanging the desk edge" makes the measurement unambiguous. Second, writing "/ cm" signals that the variable is numerical and can be given different values (10 cm, 20 cm, 30 cm, and so on). Both elements are needed.
Apply the same logic to other common setups:
Simple pendulum: IV = length of string / cm (not "length of pendulum" or "length of the string and bob")
Bouncing ball: IV = drop height / cm (not "where the ball is dropped" or "height")
Dissolving sugar: IV = temperature of water / degrees Celsius (not "temperature" alone if the unit is absent, though this is more forgiving than missing the measured property entirely)
The pattern: always name the property being varied, specify what it belongs to or where it is measured, and include the unit.
3 | How to spot each variable in a question
3.1 | "The effect of X on Y" phrasing
When a question asks you to investigate "the effect of X on Y", the structure maps directly onto the variables:
X is the IV (what is being varied to produce an effect)
Y is the DV (what is being measured to see the effect)
Example: "A student investigates the effect of the concentration of sucrose solution on the percentage change in mass of potato cylinders."
IV = concentration of sucrose solution / mol dm-3
DV = percentage change in mass of potato cylinder / %
3.2 | "Repeat with..." phrasing
When the question describes a method that says "repeat with 10, 20, 30, 40 cm" or "use three different temperatures", the thing being varied in those repeats is the IV.
Example: "The student set up the apparatus with 5 cm of ruler overhanging the bench. She then repeated the experiment with 10 cm, 15 cm, 20 cm, and 25 cm."
IV = length of ruler overhanging the bench / cm (the thing whose value changes each repeat)
DV = the result being recorded each time (e.g. time for 10 oscillations / s)
3.3 | Finding CVs: the "what else?" test
Once you have named the IV and DV, ask yourself: what other factors in this setup could affect the DV? Each answer is a potential CV.
For the ruler experiment: could the mass of the ruler affect how long it vibrates? Yes -- heavier rulers vibrate differently. Is it being changed? No -- so it is a CV. Could the material of the ruler matter? Yes. Is it being changed? No -- CV. Run this question for each feature of the experiment until you have at least four CVs.
Practice question -- try before reading the answer below:
A student investigates how the temperature of water affects the time it takes for a fixed mass of sugar to dissolve completely. She uses 50 g of sugar each time and stirs the water at the same rate.
What is the IV? What is the DV? Name two more CVs beyond the ones already mentioned.
Answer: IV = temperature of water / degrees Celsius. DV = time for sugar to dissolve completely / s. Additional CVs: volume of water (e.g. 200 cm3 each time), particle size of sugar (use the same brand and grind), whether the container is covered or open (same open beaker each time).
4 | The controlled-variables checklist
Most Sec 1 and Sec 2 experiments draw from the same pool of possible CVs. Learn this checklist and you will rarely miss one on the planning question.
Controlled variable
How to control it
Temperature of the environment or liquid
Conduct all trials at room temperature on the same day in the same room, or use a water bath set to the same temperature.
Volume of liquid used
Measure with the same measuring cylinder and use the same volume (e.g. 100 cm3) for every trial.
Concentration of solution
Prepare one stock solution and use it for all trials, or use identical pre-prepared bottles.
Mass or size of the specimen
Weigh each specimen on the same balance before the trial; cut or trim to the same dimensions using a ruler.
Starting position or drop height
Use a retort stand with the object clamped at the same measured height for every trial.
Apparatus used
Use the same measuring cylinder, thermometer, or stopwatch throughout -- do not switch instruments between trials.
Same person making the measurement
Have the same student read the scale or start/stop the stopwatch to reduce variation in reaction time.
Stirring rate (for dissolving experiments)
Use a magnetic stirrer at the same setting, or stir manually with the same number of strokes per minute.
Mark-scheme tip: the typical planning question awards one mark for naming the CV and one mark for explaining how it is controlled. Write "Temperature -- kept constant by conducting all trials in the same room on the same day" as a minimum. Vague statements like "keep temperature the same" without a method will often earn only half the available credit.
5 | Four worked examples from Sec 1 practicals
Example 1 -- Simple pendulum period
A student investigates how the length of the string affects the time for 10 complete oscillations.
Variable type
Variable
How controlled / measured
IV
Length of string / cm
Varied at 10, 20, 30, 40, 50 cm; measured with a metre rule from the pivot to the centre of the bob.
DV
Time for 10 complete oscillations / s
Measured with a stopwatch; repeated three times and averaged to reduce the effect of reaction time.
CV
Mass of the pendulum bob
Same metal bob used throughout; not exchanged between trials.
CV
Amplitude of swing (initial displacement)
Released from the same small angle (approximately 10 degrees) each time, measured with a protractor.
CV
Same stopwatch
Same stopwatch used by the same student for all timing to reduce variation from reaction time.
Example 2 -- Osmosis in potato cylinders
A student investigates how the concentration of sucrose solution affects the percentage change in mass of potato cylinders.
Variable type
Variable
How controlled / measured
IV
Concentration of sucrose solution / mol dm-3
Varied across 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol dm-3; prepared from the same stock.
DV
Percentage change in mass of potato cylinder / %
Calculated from initial and final mass measured on the same electronic balance (to 0.01 g).
CV
Initial length of each potato cylinder
Cut to exactly 4.0 cm with a scalpel and ruler; checked before immersion.
CV
Initial mass of each potato cylinder
Recorded before immersion; cylinders trimmed if they differ by more than 0.05 g.
CV
Time of immersion
All cylinders immersed for exactly 30 minutes; timed with the same stopwatch.
Example 3 -- Bounce height of a ball
A student investigates how the drop height of a ball affects how high it bounces.
Variable type
Variable
How controlled / measured
IV
Drop height / cm
Varied at 20, 40, 60, 80, 100 cm; measured from the bottom of the ball to the floor using a metre rule clamped vertically.
DV
Bounce height / cm
Measured from the floor to the highest point of the ball after the first bounce; read against the metre rule.
CV
Type of ball
Same ball used for all trials; not exchanged.
CV
Surface
All trials conducted on the same section of the same hard floor.
CV
Person releasing the ball
Same student releases the ball each time; ball dropped (not thrown) with zero initial velocity.
Example 4 -- Dissolving rate
A student investigates how the temperature of water affects the time taken for a fixed mass of salt to dissolve completely.
Variable type
Variable
How controlled / measured
IV
Temperature of water / degrees Celsius
Varied at 20, 30, 40, 50, 60 degrees Celsius; measured with the same thermometer immediately before adding the salt.
DV
Time for salt to dissolve completely / s
Timed from the moment the salt is added until no solid is visible; same student observes and stops the stopwatch.
CV
Mass of salt
5.0 g measured on the same electronic balance for every trial.
CV
Volume of water
200 cm3 measured with the same measuring cylinder each time.
CV
Stirring rate
Stirred at the same rate (one stir per second) counted aloud by the same student.
6 | The bridge to sources of error
Once you have identified your CVs, the "sources of error" question in Part (c) of the planning task becomes much more tractable. The question is simply: which CVs could NOT be controlled perfectly even with care?
Two examples:
Reaction time on the stopwatch. The DV in the pendulum experiment is the time for 10 oscillations. The student starts and stops the stopwatch by hand. Even with practice, there is a small unpredictable delay each time. This is a random error -- it is different in magnitude and direction from one trial to the next, and it cannot be eliminated, only reduced by repeating trials and averaging.
Parallax on a ruler. When reading the bounce height of the ball against a vertical ruler, the student's eye must be level with the top of the ball at its highest point. If the eye is consistently above or below that point, every reading is shifted by the same amount in the same direction. This is a systematic error -- it does not affect precision but it shifts the result away from the true value.
The full treatment of random and systematic errors, along with techniques for minimising each, is covered in the sources-of-error post in this series (Post 6 in the 2026-04-14 content slate).
7 | Common mistakes that lose marks
Naming the object instead of the measurable property. Write "drop height of the ball / cm", not "the ball" or "height". The variable must be something that can be assigned a numerical value.
Missing the unit. "Length of string / cm" earns the mark. "Length of string" often does not, because the absence of a unit suggests the student has not thought about how the variable will be measured.
No "how controlled" note on CVs. Stating "temperature" as a CV without explaining how it is controlled typically earns only partial credit. One sentence on method is enough.
Changing two things at once. If you vary both the temperature and the concentration of the solution between trials, you cannot tell which change caused the difference in result. This violates the principle of a fair test and is one of the most common planning errors at Sec 1.
Qualitative CVs. Writing "nice weather" or "same mood of the student" will not earn marks. CVs must be measurable factors that could realistically affect the outcome -- temperature, volume, mass, concentration, time, and so on.
Confusing the DV with the method of measurement. The DV is the quantity being measured ("percentage change in mass / %"), not the instrument ("using a balance"). Name the quantity and its unit first; the instrument choice belongs in the method section.
