How do you plan a fair scientific investigation?
Planning an investigation: the independent, dependent and control variables, a hypothesis, a fair test, and risk assessment.
A concise overview of the planning skills for the WJEC GCSE Science Double Award practical assessment (Unit 7), covering independent, dependent and control variables, writing a hypothesis, designing a fair test, and risk assessment.
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What this dot point is asking
The WJEC Double Award practical assessment (Unit 7) and practical questions in the written papers want you to plan a fair investigation: identify the variables, write a hypothesis, design a fair test and assess the risks.
The three types of variable
Identifying these correctly is the first step in planning, and a common exam question.
Writing a hypothesis
A good hypothesis can be tested by the investigation and is often based on scientific knowledge (here, collision theory).
Designing a fair test
You should also plan a sensible range of values for the independent variable, decide how to measure the dependent variable, and repeat readings so a mean can be taken (which improves reliability).
Risk assessment
Before any practical, a risk assessment identifies the hazards (such as hot equipment, acids or sharp objects), the risk they pose, and the precautions to reduce them (such as wearing eye protection, using tongs, or working carefully). This keeps the experimenter and others safe.
Choosing a sensible range and interval
When planning, you also decide the range (the lowest to highest values of the independent variable) and the interval (the gap between them). The range should be wide enough to show a clear pattern, and the interval small enough to give several points - usually at least five values. For example, to investigate temperature you might use 20, 30, 40, 50 and 60 degrees C. Choosing too few values, or values too close together, makes the pattern hard to see. Planning a sensible range and interval is rewarded in the practical assessment.
The specified practicals
The Double Award course includes a set of specified practicals across biology, chemistry and physics that the practical assessment and written papers draw on, such as osmosis in potato, food tests, the rate of reaction with thiosulfate, preparing a salt, investigating resistance, and specific heat capacity. Learning the standard method, the apparatus, and how the results are analysed for each of these means you can answer planning questions confidently and recognise the practical context in the written papers. Knowing these methods well is one of the best ways to prepare for Unit 7.
Try this
Q1. Which variable is the one you measure? [1 mark]
- Cue. The dependent variable.
Q2. Why are control variables kept the same? [1 mark]
- Cue. So they do not affect the results, keeping the test fair and the conclusion valid.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksA student investigates how temperature affects the rate of a reaction. Identify the independent, dependent and one control variable, and write a suitable hypothesis.Show worked answer →
A Unit 7 planning question worth 4 marks. Reward: the independent variable (the one changed) is the temperature (1); the dependent variable (the one measured) is the rate of reaction (such as time taken, or gas produced) (1); a control variable (kept the same) is the concentration or volume of the reactants (1); a suitable hypothesis is "as the temperature increases, the rate of reaction increases" (1). Markers credit the three variable types and a testable hypothesis. A common error is to confuse the independent and dependent variables.
WJEC style3 marksExplain what a fair test is and why control variables are important.Show worked answer →
A Unit 7 explain question. Reward: a fair test changes only the independent variable and keeps all other variables the same (1); the control variables are kept constant so they do not affect the results (1); this means any change in the dependent variable can be put down to the independent variable alone, making the conclusion valid (1). Markers credit changing one variable, keeping others constant, and the link to a valid conclusion. A common error is to describe a fair test as just "doing it carefully".
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