Welcome to Designing a Practical Procedure!

In Biology, we don’t just believe what people tell us—we test it! Designing a procedure is like writing a very detailed recipe. If your recipe is clear and accurate, anyone in the world should be able to follow it and "cook up" the same results. This chapter will show you how to plan a perfect investigation from scratch.

1. Starting with a Hypothesis

Before you pick up a beaker, you need a plan. Every good experiment starts with a hypothesis. This is a scientific statement that predicts a relationship between two things. It’s often based on a pattern you’ve noticed.

Example: "If I increase the light intensity, the rate of photosynthesis will increase."

Quick Tip: A hypothesis isn't just a guess; it’s a testable idea. To test it, you need to decide what you will change and what you will measure. (For more on variables, see the chapter on Variables, control and risk assessment).

2. Choosing the Right Apparatus

In your exam, you might be asked to justify why you chose a specific piece of equipment. This means giving a scientific reason for your choice. You want your measurements to be as precise as possible.

Consider these examples:

  • Measuring Volume: Instead of using a simple beaker to measure gas, you might use a gas syringe. Why? Because a gas syringe has a higher resolution (it can measure smaller differences, like \( 0.1 \text{ cm}^3 \)).
  • Measuring Temperature: A digital thermometer is often better than a glass one because it is easier to read and less likely to involve human error.
  • Timing: A stopwatch is essential for measuring the rate of a reaction, such as how long it takes for an enzyme to digest starch.

Key Takeaway: Always choose equipment that gives you the most accurate and precise results!

3. Writing the Method: Step-by-Step

A good procedure should be a numbered list of instructions. Don't worry if this seems tricky at first—just imagine you are explaining it to a friend who has never been in a science lab before.

A perfect method includes:

  1. Clear steps: Use command words like "Measure," "Place," and "Record."
  2. Specific values: Don't just say "add some water." Say "add \( 20 \text{ cm}^3 \) of water."
  3. The range and interval: If you are testing temperature, say "test at \( 10^\circ\text{C}, 20^\circ\text{C}, 30^\circ\text{C}, 40^\circ\text{C}, \text{ and } 50^\circ\text{C} \)." (In this case, the range is \( 10-50 \) and the interval is \( 10 \)).
  4. The number of repeats: You should always state that you will repeat the experiment at least three times for each value to calculate a mean.

4. Repeatability, Reproducibility, and Validity

These are "big" words that often confuse students, but they are quite simple once you know the trick!

  • Repeatable: If you do the experiment again using the same method and equipment, do you get the same results?
  • Reproducible: If someone else (or you use different equipment/techniques) does the experiment, do the results still match?
  • Valid: Is your experiment a "fair test"? Does it actually test what it’s supposed to? To be valid, you must keep all your control variables the same.

Common Mistake: Students often use the word "reliable." In the OxfordAQA 9201 syllabus, we don't use "reliable"—we use repeatable and reproducible instead!

5. Evaluating and Improving

No experiment is perfect! At the end of your design, you should suggest improvements. This shows you are thinking like a real scientist.

How to suggest improvements:
  • To increase precision: "Use a measuring cylinder with smaller scale divisions."
  • To identify anomalies: "Repeat the experiment more times to ensure the results are consistent."
  • To reduce errors: "Use a water bath to keep the temperature constant (control variable) more effectively."

6. Practicals in Focus

In your Paper 2 exam, you are specifically expected to know how to design procedures for these two areas:

  • Photosynthesis: Designing a way to see how light intensity affects the rate (e.g., moving a lamp different distances from pondweed and counting bubbles).
  • Exercise: Designing a way to see how exercise affects the human body (e.g., measuring heart rate before and after different intensities of running).

Did you know? When counting bubbles in the photosynthesis practical, the bubbles can be different sizes! To make the procedure more valid, you could collect the gas in a capillary tube and measure the length of the gas bubble instead of just counting them.

Quick Review:
1. Start with a testable hypothesis.
2. Justify your apparatus (choose the best tool for the job).
3. Write a clear, numbered method.
4. Include repeats (at least 3) to find a mean.
5. Check for validity by controlling variables.