Key Terms: Experimental Design
| Experiment | A scientific test done in a controlled way to prove or disprove a hypothesis. |
| Hypothesis | An educated guess or a testable statement predicting the outcome of an experiment. It usually follows an "If... then..." format. |
| Variable | Anything that can change or be changed in an experiment. |
| Independent Variable | The factor that a scientist changes on purpose in an experiment. It is the 'cause'. |
| Dependent Variable | The factor that is measured or observed during an experiment. It is the 'effect' that responds to the independent variable. |
| Controlled Variables | Factors that are kept the same throughout the experiment to ensure a fair test. |
| Control Experiment | A setup where the independent variable is either absent or kept at a 'normal' level. It is used for comparison with the experimental group. |
| Reliability | The consistency of measurements. An experiment is reliable if you get similar results when you repeat it many times. |
| Validity | The extent to which an experiment measures what it is intended to measure. A valid experiment gives accurate and unbiased results. |
Figure: Key terms and definitions in experimental design
KEY PRINCIPLES FOR EXPERIMENTAL DESIGN Since this topic is not about formulas, here are the key principles you must follow when designing an experiment: • State the Problem Clearly: Know exactly what question you want to answer. • Formulate a Hypothesis: Make a testable prediction. For example, "If I give maize plants more water, then they will grow taller." • Identify Variables: Clearly state the independent, dependent, and controlled variables. • Design a Control Experiment: Always include a setup where the independent variable is not applied, so you have something to compare your results to. • Repeat the Experiment (Replication): Do your experiment multiple times (e.g., use several maize plants in each group). This makes your results more reliable. • Collect Data Accurately: Measure and record your observations carefully. • Analyse Data and Draw Conclusions: Look at your results and decide if they support or reject your hypothesis. MUST-KNOW FACTS 1. Purpose of an Experiment: To test a hypothesis or answer a scientific question. 2. Only One Independent Variable: In a fair test, you must change only one thing at a time (the independent variable). 3. Keeping Things the Same: All other factors (controlled variables) must be kept constant. If they change, your results will not be clear. 4. Control Group Importance: The control group acts as a baseline. It shows what happens naturally without the change you are testing. 5. Role of Repetition: Repeating an experiment and taking an average helps to reduce the effect of random errors. This makes your results more reliable. 6. Safety First: Always consider safety precautions before, during, and after an experiment. This includes wearing safety glasses, handling chemicals carefully, and proper waste disposal. 7. Measurements: Use appropriate measuring tools (e.g., ruler for length, measuring cylinder for volume, stopwatch for time) and record units correctly. 8. Data Recording: Present your observations clearly, often in a table, before you can analyse them. 9. Conclusion: Your conclusion should directly address your hypothesis and be supported by your experimental data. 10. Limitations: Good scientists also recognise any problems or limitations in their experiment and suggest improvements.EXPERIMENTAL DESIGN: PLANT GROWTH |
Figure: Diagram of an experiment to test the effect of water on plant growth
COMPARISON TABLEComparing Types of Variables
| Variable Type | Description | Example (in plant growth experiment) |
|---|---|---|
| Independent Variable | What the scientist changes on purpose. | Amount of water given to each plant. |
| Dependent Variable | What is measured or observed as a result of the change. | Height of the plant, number of leaves. |
| Controlled Variables | Things kept the same to make the test fair. | Type of plant, type of soil, amount of sunlight, temperature, pot size. |
Figure: Comparison of different types of variables
COMMON EXAM QUESTIONS & MODEL ANSWERS 1. Question: State three characteristics of a good scientific experiment. (3 marks) Model Answer: * It should only change one variable at a time (independent variable). * It should include a control experiment for comparison. * It should be repeated multiple times to ensure reliable results. 2. Question: A student wants to investigate the effect of different types of fertiliser on the growth of groundnut plants. (a) State a suitable hypothesis for this experiment. (1 mark) (b) Identify the independent variable. (1 mark) (c) Identify the dependent variable. (1 mark) (d) Suggest two controlled variables for this experiment. (2 marks) Model Answer: (a) If different types of fertiliser are used, then they will affect the growth of groundnut plants differently. (b) Type of fertiliser. (c) Growth of groundnut plants (e.g., plant height, mass, yield). (d) Type of groundnut seed, amount of sunlight, amount of water, type of soil, temperature, pot size. (Any two) 3. Question: Explain the importance of a control experiment in a scientific investigation. (2 marks) Model Answer: A control experiment provides a baseline for comparison. It shows what happens when the independent variable is not applied, helping to ensure that any observed changes are indeed due to the independent variable. 4. Question: Why is it important to repeat an experiment multiple times? (2 marks) Model Answer: Repeating an experiment helps to make the results more reliable. It reduces the impact of random errors or unusual results from a single trial, giving a more accurate average. 5. Question: Outline the basic steps of the scientific method. (4 marks) Model Answer: 1. Make an observation and ask a question. 2. Formulate a hypothesis (a testable prediction). 3. Design and conduct an experiment to test the hypothesis. 4. Collect and analyse data from the experiment. 5. Draw a conclusion that supports or rejects the hypothesis.Steps of the Scientific Method
| 1. Observation / Question |
| ↓ |
| 2. Formulate Hypothesis |
| ↓ |
| 3. Design & Conduct Experiment |
| ↓ |
| 4. Collect & Analyse Data |
| ↓ |
| 5. Draw Conclusion |
Figure: Flowchart of the scientific method
MEMORY AIDS & MNEMONICS • DRY MIX for variables: * Dependent variable Responding (what you You measure). * Manipulated variable Independent (X you change). • CHORD for experiment steps: * Control * Hypothesis * Observations * Replication * Data & Conclusion • "Only One Changer": Remember to only change one independent variable in a fair test. COMMON MISTAKES TO AVOID 1. Changing Too Many Things: Do not change more than one independent variable at a time. If you do, you won't know which factor caused the result. 2. No Control Group: Forgetting to include a control experiment means you have nothing to compare your results to. You cannot be sure your changes caused the effect. 3. Not Controlling Other Variables: Failing to keep all other factors constant makes the experiment unfair. The results will not be valid. 4. Not Repeating the Experiment: Relying on a single trial can lead to unreliable results because of random errors or unexpected events. 5. Confusing Variables: Mixing up the independent and dependent variables. Remember, the independent variable is what you change, and the dependent variable is what you measure. 6. Unclear Hypothesis: A hypothesis must be a testable prediction, not just a general statement. Make sure it can be proven or disproven by your experiment. LAST-MINUTE CHECKLIST ☐ Can I define hypothesis, independent variable, dependent variable, and controlled variables? ☐ Can I explain why a control experiment is important? ☐ Can I explain why repeating an experiment makes results more reliable? ☐ Can I identify the different variables in a given experimental scenario (e.g., testing temperature effect on seed germination)? ☐ Can I list at least three controlled variables for a plant growth experiment? ☐ Can I state the main steps of the scientific method? ☐ Do I know why it is important to change only one variable at a time? ☐ Can I give an example of a safety precaution in a biology experiment? ☐ Can I suggest ways to make an experiment fair and reliable?