Designing is where your question becomes a plan. The test is simple: could another student pick up your method, follow it without asking you a single question, and get the same kind of data? If not, it needs more detail.
📚 What you need to know
List and justify your variables: the independent one you change, the dependent one you measure, and the controlled ones you keep constant.
Use at least five values of the independent variable so a trend can appear, and say why you chose that range.
Repeat every value at least three times, and five is better in biology because living material varies so much.
Repeats let you calculate a mean, which reduces the effect of random error and makes anomalies obvious.
Write the method as materials, then safety, ethical and environmental, then a numbered procedure.
Give exact quantities and apparatus, not “some” and “a bit”.
Investigations can also use databases, simulations or surveys, not just hands-on lab work.
A pilot study is a small trial run that saves you from discovering problems too late.
The three kinds of variable
Every design starts here, and every one of them needs a reason attached.
Variable
What it is
Example: light and pondweed
Independent (IV)
The single thing you deliberately change
Distance of the lamp: 10, 20, 30, 40, 50 cm
Dependent (DV)
The thing you measure, to see the effect
Oxygen bubbles released per minute
Controlled (CVs)
Everything else that could plausibly change the result
Water temperature, length of pondweed, sodium hydrogencarbonate concentration, time left to settle
Controlled variables are the ones that decide whether your investigation is valid. If the water heats up because the lamp is close, you are no longer measuring the effect of light alone — you are measuring light and temperature mixed together, and you cannot separate them afterwards.
Do not just list controlled variables. For each one, say how you will hold it constant and why it matters. “Temperature: kept at 25 °C in a water bath, because temperature affects the enzymes of photosynthesis” is worth far more than the word “temperature” on its own.
How many values, how many repeats
Two numbers to justify, and the reasoning is different for each.
Range and number of values. Five different values of the IV is the working minimum — with fewer, you cannot tell a straight line from a curve. The range must cover the part where something interesting happens. Choose it from your background reading, not at random.
Repeats. Three trials at each value is the minimum; five is better in biology. Living things vary: two potato cylinders from the same potato are not identical, and neither are two leaves from the same plant. Repeats let you take a mean, which cancels out some of that random variation and makes a genuinely odd reading stand out as an anomaly.
Sketching this grid before you start tells you how long the practical will take — and whether you have enough beetroot, pondweed or lesson time to finish it.
Justifying a range, in one sentence. “A range of 20 to 60 °C was chosen because below 20 °C the reaction is too slow to time accurately, and above 60 °C the enzyme is expected to be fully denatured, so this range should capture the optimum.” Notice that both ends are justified, not just the middle.
Writing a method someone else could follow
A method is a numbered set of instructions precise enough to be repeated exactly. The usual weakness is vagueness.
Not “add some hydrogen peroxide”, but “add 5.0 cm3 of 2% hydrogen peroxide using a 10 cm3 graduated pipette”.
Not “cut up a potato”, but “use a size 5 cork borer to cut cylinders, then trim each to 3.0 cm with a scalpel and ruler”.
Not “leave it for a while”, but “leave for 20 minutes in a water bath at 30 °C”.
Quantities, concentrations, apparatus sizes, and times — all of them stated.
“Handle chemicals with care” scores nothing. “Hydrogen peroxide is an irritant, so wear goggles and gloves and rinse spills with water” names a hazard and a response.
Creativity counts. Marks are available for solving a measurement problem well. Timing a colour change by eye is subjective; using a colorimeter to measure absorbance gives you a number, and a better one. Counting bubbles is rough; collecting the gas in a syringe and reading a volume is better. Think about how you could measure your DV more objectively.
Not every investigation happens at a bench
Hands-on lab work is the most common route, but it is not the only one.
Databases. Pull real data from an established source — gene sequences from GenBank, conservation status from the IUCN Red List, climate or ecological records — then process and analyse it to find a trend.
Simulations. Useful for processes too slow or too complex for a school lab, such as population dynamics, natural selection over many generations, or predator–prey cycles.
Surveys. A well-designed questionnaire can gather data on human physiology, diet or ecological awareness — as long as the questions are consistent and the sample is reasonable.
These still need the same things: a focused question, a defined IV and DV, and a plan for how the data will be collected and handled.
Run a pilot first
A pilot study is a quick, small-scale trial of your method. It exists to find problems while they are still cheap to fix.
A single trial run tells you whether your reaction finishes in 10 seconds or 40 minutes, whether the colour change is visible at all, and whether your chosen range shows any difference. If the reaction is too fast, you dilute the enzyme; if nothing happens at the low end, you shift the range. Far better to discover that in one lesson than after three.
Write your pilot up briefly and say what you changed because of it. “The pilot showed the reaction was complete in under 15 seconds, so the enzyme was diluted from 2% to 0.5%” is exactly the kind of sentence that shows you designed the method rather than copying it.
Worked examples
WORKED EXAMPLE 1
Designing a membrane permeability investigation: “What is the effect of temperature (30 to 70 °C) on pigment leakage from beetroot discs?” Identify and justify the variables.
Independent variable
Temperature of the water bath: 30, 40, 50, 60 and 70 °CFive values, spread evenly, chosen because membrane damage is expected to begin somewhere in the middle of this range.Dependent variableAbsorbance of the surrounding water at 530 nm, using a colorimeterMore pigment leaked means a darker solution and a higher absorbance. A colorimeter gives a number instead of an opinion about the shade of red.Controlled variables and how
Same beetroot for every disc — natural variation between beetroots
Same cork borer size and 3.0 cm length — keeps surface area constant, and leakage depends on surface area
Same volume of distilled water (20.0 cm3) — the same pigment in less water looks more concentrated
Same immersion time (10 minutes, timed) — longer means more leakage regardless of temperature
Discs rinsed before use — removes pigment released by cutting, which would otherwise inflate every reading
One IV, one measurable DV, five CVs each with a method and a reason
WORKED EXAMPLE 2
A student plans three temperatures with two repeats each, and finishes in one lesson. Explain two problems with this design and suggest improvements.
Problem 1: too few values of the IVThree points cannot show the shape of a trendThree points can be joined by a straight line or a curve equally well, so an optimum could sit between two of them and never be seen.Improvement
Use at least five values across the range
Problem 2: too few repeatsTwo readings give a weak mean and no way to spot an anomalyWith two values that disagree, you cannot tell which one is the odd one out.Improvement
Repeat each value at least three times, ideally five, because biological material varies naturally
5 values × 5 repeats = 25 readings, not 6If that is too many for the time available, narrow the question — do not thin out the data.
💡 Exam tip
Justify, do not just list. Every variable, every range and every repeat count needs a reason next to it.
Justify both ends of your range, not just the middle.
Give apparatus with its precision — “a 10 cm3 graduated pipette”, “a balance reading to 0.01 g”.
Write safety as its own section, with named hazards and specific precautions.
Say why you chose that organism as well as that method.
Mention your pilot and what it changed.
⚠ Common mix-up
Confusing IV and DV. You change the IV; you measure the DV.
Listing controlled variables with no method. Naming temperature is not controlling it.
Using three points and calling it a trend. Five is the working minimum.
Treating repeats as optional when time is short. They are what makes the mean meaningful.
Writing “wear goggles” as the whole safety section. Name the actual hazard in your experiment.
Copying a method from a textbook without adapting it to your question, your organism and your range.
Up next: Controlling Variables — calibration, constant conditions, representative samples and control runs, which is where a good design becomes valid data.
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