How do you plan a fair investigation in IGCSE science?

THE SHORT ANSWER

What makes an investigation a fair test?

A fair test deliberately changes one independent variable, measures a dependent variable and keeps other relevant conditions controlled. It also uses a method someone else could follow, enough repeated measurements to judge consistency, suitable units and an evaluation of limitations. A clear plan explains why each control matters instead of writing “keep everything else the same”.

In a planning question, start with the relationship you want to test. Write the independent variable, dependent variable, at least two important controls, measurement method, repeats and one sensible safety consideration. Use your own exam board's current syllabus and mark scheme to check what the question actually requires.

BEFORE CHOOSING EQUIPMENT

Independent, dependent and controlled variables: which is which?

1. Independent variable: the one factor you deliberately change between trials. Describe a range and units where possible.
2. Dependent variable: the outcome you measure to test the question. State the instrument or observable quantity, including units.
3. Controlled variables: conditions that could influence the outcome and should remain constant. Name exactly how you would control them, not just that you would.
4. Repeated trials: measurements at each setting that can reveal inconsistent results. A repeated reading is not the same as secretly changing the method midway through the test.
5. Safety and limitations: identify a relevant hazard and a realistic mitigation, then explain a major measurement limitation without inventing equipment accuracy.

ORIGINAL WORKED PHYSICS EXAMPLE

How could you investigate whether pendulum length affects its period?

Question: How does changing the length of a simple pendulum affect the time of one complete oscillation? This is an original teaching scenario, not a Cambridge past-paper task or a prescribed practical. The procedure is described for understanding and planning; any real investigation belongs in an appropriately supervised learning environment.

1. Define the variables and controls

Independent variable: pendulum length, measured from the suspension point to the bob's centre, for example 0.20 m, 0.50 m and 0.80 m. Dependent variable: time for ten complete oscillations in seconds, then divide by ten to estimate the period. Controls: use the same bob and release angle and start each trial without pushing; measure length consistently from the same reference points.

2. State a repeatable measurement method

In a supervised school laboratory, the learner would measure a chosen length, make a small release without pushing, count ten complete oscillations and time them. Repeat the measurement at the same length rather than relying on one stopwatch reading. Repeat for the other lengths using the same counting rule, bob and release angle. Longer counting intervals can reduce the relative effect of one reaction-time error, but do not eliminate it.

3. Present and interpret the results

The following numbers are illustrative invented data, not experimental observations. Suppose the time for ten oscillations was 9.0 s at 0.20 m, 14.2 s at 0.50 m, and 17.9 s at 0.80 m. Divide each by ten: the estimated periods would be 0.90 s, 1.42 s and 1.79 s. In this illustrative set, the period becomes longer as pendulum length increases. A real conclusion would require actual repeated measurements and consideration of uncertainties.

4. Evaluate the method honestly

Human reaction time can affect timing, miscounting can change the result, and inconsistent release angles can compromise comparison. A more reliable approach uses repeated trials, a consistent reference mark and, if the school provides it, a suitable electronic timing method. It is more precise to say “these measurements suggest a longer period at greater lengths” than “the experiment proves the rule for every pendulum”.

Safety note: suspended equipment must be secure and the swing area kept clear; a teacher should assess the setup. Online tutoring covers the underlying reasoning and calculations, not supervised physical laboratory provision.

TRANSFER THE METHOD

Does the same planning logic work in Biology and Chemistry?

Biology 0610: for a hypothetical study of how temperature influences an enzyme-controlled process, specify a response you can measure, how temperature would be controlled and why timing and sample quantities must remain consistent. Do not assume an unsafe home experiment is appropriate. Chemistry 0620: for a hypothetical comparison of reaction rates, identify the chosen concentration as the independent variable, an observable rate measure as the dependent variable, and controls such as temperature, volumes and particle size where relevant. Actual reagents and safe procedures require a qualified educator's instructions.

The scientific reasoning transfers; the exact practical components, permitted apparatus and marking requirements do not necessarily transfer across subjects, boards or examination years. Always check the learner's actual syllabus and school-provided method before planning assessed work.

TRY IT YOURSELF

A six-point experimental-planning checklist

1. Write the research question as “How does changing X affect Y?”
2. Identify X (independent) and Y (dependent), including a valid measurement scale and unit.
3. Explain at least two controls and how each is maintained, naming the source of possible unfairness.
4. Describe enough apparatus and repeatable steps that another student understands what to measure.
5. State a sensible number of repeats, the recording method, and how results could be compared fairly.
6. Add a relevant hazard control, one plausible limitation and one realistic improvement.

Check your plan: Could a second learner repeat the method using your instructions, record comparable results and understand which quantities are estimated rather than observed? If not, rewrite the ambiguous step. It is better to give a clear, specific method than a long list of equipment with no reasoning.

OFFICIAL CURRICULUM CHECKS

Which Cambridge sources should you use?

Cambridge provides separate syllabuses for Biology 0610, Chemistry 0620 and Physics 0625. Cambridge has also published an educational planning-experiments resource for all three sciences. Verify the syllabus code and the version that applies to the actual examination year. Those resources are for curriculum verification; the examples and wording on this page are independently written.

This is an independent tutoring resource by Mohammed Ehab ElNomany, not official Cambridge material, and no Cambridge affiliation, practical-laboratory provision, exact grade or examination result is promised.