Planning an investigation

Q: What are the three main types of variables in a scientific investigation?

The three main types are the independent variable (what you change), the dependent variable (what you measure), and control variables (what you keep constant).

Q: What is the definition of resolution in measuring instruments?

Resolution is the smallest change in a quantity being measured that gives a detectable change in the instrument's reading.

Q: What is the difference between validity, reliability, and accuracy in an experiment?

Validity means only the IV affects the DV (fair test); reliability means repeats yield consistent results; accuracy means measurements are close to the true value.

Q: What three components are required in a complete experimental risk assessment?

A complete risk assessment requires naming the hazard , stating the risk (how it can cause harm), and providing a control measure (precaution to reduce risk).

Science Double Award 1370 · CCEA GCSE · 7 min read · Updated: 31 Aug 2026

Welcome to Planning an Investigation!

Have you ever wondered how scientists discover new medicines, test racing cars, or find the best recipe for plant fertilizer? It all starts with a solid plan. In Unit 7: Practical Skills, learning how to plan an investigation is one of the most valuable skills you will develop. Not only does it help you think like a real scientist, but it also earns you major marks in your GCSE exams!

Don't worry if planning an experiment feels a bit overwhelming at first. We are going to break it down into simple, manageable steps so you can tackle any practical question with total confidence.

1. Identifying Variables: The Three Pillars

Every good experiment is designed to find out how one factor affects another. In science, we call these factors variables. There are three types you need to know inside out:

• Independent Variable (IV): This is the variable that YOU choose to change. Think: "I change the Independent variable." For example, if you are testing how temperature affects how fast sugar dissolves, the temperature of the water is your independent variable.

• Dependent Variable (DV): This is the variable that you measure or observe. It changes in response to the independent variable. Think: "Dependent is the Data you collect." In the sugar experiment, the time taken for the sugar to dissolve completely is the dependent variable.

• Control Variables (CV): These are all the other factors that you must keep the same throughout the experiment. If you don't keep them constant, your test will not be fair! In our sugar test, control variables would include the volume of water, the mass of sugar, and the speed of stirring.

Analogy: Making the Perfect Toast

Imagine you want to find the perfect toaster setting. The setting on the dial is what you change (Independent Variable). How brown or crispy the bread becomes is what you observe (Dependent Variable). To make it a fair test, you must use the exact same type, brand, and thickness of bread every time (Control Variables).

Common Mistake to Avoid

Students often write: "Keep everything else the same." In an exam, this is too vague! Always name at least two specific control variables (e.g., "Keep the volume of acid at \(25\ \text{cm}^3\) and use the same mass of magnesium ribbon each time").

Key Takeaway: Change only one thing (IV), measure the outcome (DV), and keep everything else constant (CV) to ensure a fair test.

2. Writing a Hypothesis and Making Predictions

Before jumping into an experiment, a scientist proposes an idea called a hypothesis. A hypothesis is a testable statement explaining what you think will happen and why.

• A Good Hypothesis: Clearly links the independent variable to the dependent variable.
Example: "As the temperature of the acid increases, the rate of reaction will increase."

• Providing Scientific Reasoning: Always back up your prediction with scientific theory.
Example: "...because the particles will have more kinetic energy, move faster, and collide more frequently and successfully."

Key Takeaway: A scientific prediction follows the pattern: "If I increase/decrease [IV], then [DV] will increase/decrease because [scientific reason]."

3. Choosing the Right Equipment

To get reliable results, you must choose equipment suitable for measuring your variables accurately.

• Measuring Volumes of Liquids: A measuring cylinder, pipette, or burette is used. Never use a beaker to measure liquid volumes accurately, as the markings on beakers are only rough estimates!

• Measuring Mass: Use an electronic balance (often measuring to \(0.01\ \text{g}\) or \(0.1\ \text{g}\)).

• Measuring Time: Use a digital stopwatch or timer (measuring in seconds).

• Measuring Temperature: Use a thermometer or a digital temperature probe (measuring in \(^\circ\text{C}\)).

• Measuring Length: Use a metre ruler, measuring tape, or vernier callipers depending on the scale.

Did You Know?

Resolution is the smallest change a measuring instrument can detect. For example, a standard school ruler has a resolution of \(1\ \text{mm}\), whereas a digital balance might detect a tiny change of \(0.01\ \text{g}\). Choosing instruments with a higher resolution gives you more detailed and precise measurements!

Key Takeaway: Always name specific pieces of apparatus and select instruments that measure with the right degree of accuracy.

4. Designing a Step-by-Step Method

When asked to write an experimental method in an exam, write it in clear, logical, numbered steps. Anyone reading your method should be able to carry out the experiment exactly as you did.

The 5-Point Method Checklist

1. Apparatus Setup: State clearly how the equipment is assembled and what values you start with.
2. Independent Variable Range: Choose at least 5 different values across a sensible range (for example: testing temperatures of \(20\ ^\circ\text{C}\), \(30\ ^\circ\text{C}\), \(40\ ^\circ\text{C}\), \(50\ ^\circ\text{C}\), and \(60\ ^\circ\text{C}\)).
3. Measurement (Dependent Variable): Explain precisely what you are measuring and what instrument you are using (e.g., "Measure the time in seconds using a digital stopwatch").
4. Control Variables: State which variables are kept constant and specify how you keep them constant.
5. Repetition and Averages: State that you will repeat the experiment 3 times at each setting to calculate a mean (average) and identify any anomalies (outliers).

Calculating a Mean

To find the mean of repeated trials:

\(\text{Mean} = \frac{\text{Trial 1} + \text{Trial 2} + \text{Trial 3}}{3}\)

Note: If one value is clearly an anomaly (very far off the others due to an error), leave it out before calculating the mean!

Key Takeaway: A complete method includes the setup, a range of at least 5 values, named control variables, and repeating at least 3 times for a mean.

5. Ensuring Reliability, Validity, and Accuracy

These three terms are frequently tested. Let's make them crystal clear:

• Validity: Does the experiment truly test what it is supposed to test? An experiment is valid if you only change the independent variable while keeping all control variables constant.

• Reliability (Repeatability): Can the results be reproduced? If you repeat the test multiple times and get consistent results (similar values), your data is reliable.

• Accuracy: How close a measurement is to the true or accepted value. Using high-resolution equipment and correct measuring techniques (like reading at eye level to avoid parallax error) improves accuracy.

Key Takeaway: Validity is about a fair test; Reliability is about consistent repeats; Accuracy is about getting close to the true value.

6. Safety and Risk Assessment

No scientific investigation is complete without considering safety. In exam questions, a full risk assessment requires three parts:

1. The Hazard: The object or chemical that can cause harm (e.g., "Hot water", "Corrosive acid", "Glass beaker").
2. The Risk: What could go wrong or how the hazard causes injury (e.g., "Hot water could spill and cause burns or scalds to the skin").
3. The Control Measure (Precaution): The action taken to reduce the risk (e.g., "Wear heatproof gloves and handle the beaker with care" or "Wear safety goggles to protect eyes from splashes").

Quick Lab Safety Rules

• Always wear safety goggles when heating substances or using chemicals.
• Tie long hair back and tuck in ties when working near a Bunsen burner.
• Stand up while carrying out practical work so you can move away quickly if a spill occurs.
• Report any broken glassware immediately to the teacher.

Key Takeaway: Always link the Hazard \(\rightarrow\) Risk \(\rightarrow\) Control Measure in your safety explanations.

Quick Review: The 6-Mark Planning Checklist

Whenever you are faced with an exam question asking you to "Plan an investigation to...", run through this quick mental checklist:

• I - Independent Variable: Did I state what I will change and give at least 5 specific values/ranges?
• D - Dependent Variable: Did I state what I will measure and name the measuring instrument?
• C - Control Variables: Did I name at least 2 specific variables to keep constant and how to do so?
• R - Repeats: Did I mention repeating each test 3 times to calculate a mean?
• S - Safety: Did I identify a hazard, the risk it poses, and a sensible precaution?

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What are the three main types of variables in a scientific investigation?

The three main types are the independent variable (what you change), the dependent variable (what you measure), and control variables (what you keep constant).

What is a scientific hypothesis and what structure should it follow?

A hypothesis is a testable statement linking the independent and dependent variables, supported by scientific reasoning (e.g., "If [IV] increases, then [DV] increases because [scientific reason]").

Why should measuring cylinders, pipettes, or burettes be used instead of beakers to measure liquid volumes?

Beakers only have approximate markings, whereas measuring cylinders, pipettes, and burettes are calibrated for accurate and precise volume measurements.

What is the definition of resolution in measuring instruments?

Resolution is the smallest change in a quantity being measured that gives a detectable change in the instrument's reading.

What are the key elements required when writing a step-by-step experimental method?

A method must include apparatus setup, a range of at least 5 independent variable values, the dependent variable and its measuring tool, control variables, and repeating 3 times to calculate a mean.

How is a mean calculated from repeated trials, and what should be done with anomalies?

Sum the results of the concordant trials and divide by the number of trials; any clear anomalies (outliers) must be excluded before calculating the mean.

What is the difference between validity, reliability, and accuracy in an experiment?

Validity means only the IV affects the DV (fair test); reliability means repeats yield consistent results; accuracy means measurements are close to the true value.

What three components are required in a complete experimental risk assessment?

A complete risk assessment requires naming the hazard, stating the risk (how it can cause harm), and providing a control measure (precaution to reduce risk).

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