Introduction to Planning Experiments

Welcome to one of the most important chapters for your Unit 5 exam! While you don’t have a practical "lab exam" in the OxfordAQA International A-level, you will be asked to plan experiments on paper. This chapter focuses on how to design a "fair test" by identifying what you change, what you measure, and how you keep everything else the same. Mastering these skills is essential for tackling Assessment Objective 4 (AO4) questions, which make up a significant portion of your marks.

1. Identifying the Variables

In any biological investigation, there are three main types of variables you must be able to identify and describe. Don't worry if these seem basic; the key is being specific in your descriptions.

The Independent Variable (IV)

The independent variable is the factor that you intentionally change to see what happens.
Example: If you are investigating the effect of temperature on the rate of respiration in yeast (Practical 8), the temperature is your independent variable.

Top Tip: In your exam, always state the range and intervals of your IV. For example: "Use five different temperatures (\(20^{\circ}C\), \(30^{\circ}C\), \(40^{\circ}C\), \(50^{\circ}C\), and \(60^{\circ}C\)) using a thermostatically controlled water bath."

The Dependent Variable (DV)

The dependent variable is the factor that you measure as a result of changing the IV.
Example: In the yeast experiment, the DV might be the volume of carbon dioxide produced in a set time or the time taken for a methylene blue indicator to turn colorless.

Control Variables (CV)

Control variables are all the other factors that could affect your results. You must keep these constant so that any change in your DV is definitely caused by your IV.
Example: When testing enzymes (Practical 1), you must keep pH constant (using a buffer solution) and the concentration of substrate the same.

Key Takeaway: If you don't control these variables, your experiment is not a "fair test," and your results will be invalid.

2. Planning a Valid Method

When asked to describe a method, imagine you are writing a recipe for someone who has never done biology before. It needs to be step-by-step and logical.

Setting the Range and Intervals

You need enough data points to see a trend. Usually, at least five different values for your independent variable are required.
Why? If you only test two temperatures, you can only draw a straight line. With five, you can see if the relationship is a curve (like an enzyme activity curve) or if it levels off.

Replicates and Reliability

Biological material is notoriously "fussy." To ensure your results are reliable, you should:
1. Repeat the experiment at each level of the IV (at least three times).
2. Calculate a mean (average) from these repeats.
3. Identify any anomalies (results that don't fit the pattern) and exclude them from your mean calculation.

The "Control" Experiment

Note the difference between control variables and a control experiment. A control experiment is a parallel setup where the IV is removed or replaced with something inactive (like boiled/denatured enzymes or distilled water).
Purpose: To prove that the independent variable alone is causing the effect you see.

3. Choosing the Right Equipment

In Unit 5, you are expected to select equipment that provides the best accuracy and precision.

Accuracy: How close a measurement is to its "true" value.
Example: Using a gas syringe to measure gas volume is more accurate than counting bubbles, because bubbles vary in size.

Precision: Using equipment with smaller scale divisions (higher resolution).
Example: Using a graduated pipette (divisions of \(0.1 \ cm^{3}\)) rather than a measuring cylinder (divisions of \(1.0 \ cm^{3}\)).

Helpful Analogy: Imagine throwing darts. If they all land close together but far from the bullseye, you are precise but not accurate. If they are all near the bullseye, you are both!

4. Controlling Environmental Factors (The "How-To" Guide)

Struggling to remember how to control variables? Use this quick reference list:

  • Temperature: Use a thermostatically controlled water bath. (Avoid just saying "a bowl of water").
  • pH: Use a buffer solution. These solutions resist changes in pH.
  • Light Intensity: Place a lamp at a fixed distance, measured with a metre ruler, in a dark room to avoid interference from sunlight.
  • Concentration: Use serial dilutions to create a range of known concentrations from a "stock" solution.
  • Time: Use a stopwatch or digital timer and state a specific duration (e.g., "leave for 5 minutes").

5. Safety and Risk Assessment

Every plan needs a safety check. You should identify a hazard, the risk it poses, and a control measure to minimize that risk.

Common Hazards in Biology (9610):
1. Scalpels/Glassware: Risk of cuts. Control: Cut away from fingers; keep glassware away from table edges.
2. Chemicals (e.g., Benedict's reagent, Acids): Irritants or Corrosive. Control: Wear safety goggles and gloves.
3. Heat: Risk of burns. Control: Use tongs to handle hot test tubes.
4. Microorganisms: Risk of infection. Control: Use aseptic techniques (like flaming loops and disinfecting surfaces).

Quick Review: The Planning Checklist

Before you finish a planning question, check if you have included:

\( \bullet \) The Independent Variable (What you change, the range, and the units).
\( \bullet \) The Dependent Variable (What you measure and the equipment used).
\( \bullet \) At least two Control Variables (What you keep the same and how you do it).
\( \bullet \) A Control Experiment (To show the IV is responsible).
\( \bullet \) Repeats (To calculate a mean and improve reliability).
\( \bullet \) Safety (At least one hazard and a way to reduce the risk).

Note: For more details on the specific "Required Practicals" mentioned in the syllabus, please see the dedicated chapters for Practicals 1-10. This chapter focuses strictly on the general principles of planning applicable to all of them.