1.1.1(a)-(c) - Planning experimental design and controlled variables
In this lesson you are learning how to turn a biological question into a practical method that can actually answer it. The skill is not just naming variables; it is choosing apparatus, techniques and controls so the results could meet the expected outcome. OCR often tests this by giving you a practical context and asking whether the plan is suitable, what should be controlled, or why a method detail matters.
The Purpose of Experimental Design
Experimental design is the planning of a practical investigation so that the method can answer a scientific question. In Biology, the question usually asks whether changing one factor affects a biological response.
Experimental Design
The selection of a suitable method, apparatus, equipment, measurements and controls to answer a practical biological question.
A good design starts with a clear expected outcome. The expected outcome is not always "the result will definitely be higher"; it is what the method must be able to detect. For example, if a student investigates whether temperature affects enzyme activity, the method must produce a measurable rate at different temperatures. If the method only records "some bubbles formed", it is too vague to test a rate properly.
Most planning answers depend on three questions:
| Planning question | What it checks |
|---|---|
| What will be changed? | The independent variable is clear and deliberately varied. |
| What will be measured? | The dependent variable gives evidence for the outcome. |
| What else could affect the outcome? | Important controlled variables are kept constant or accounted for. |
Independent Variable
The factor deliberately changed by the investigator.
The dependent variable is different because it is not chosen as the treatment; it is the evidence collected from each treatment.
Dependent Variable
The factor measured or observed to judge the effect of the independent variable.
In exam answers, be precise. "Amount" is often too vague. "Volume of amylase solution", "concentration of sucrose solution" or "distance moved by an air bubble in 5 minutes" is much more creditworthy because it tells the reader exactly what is being controlled or measured.
The Purpose of Experimental Design Continued
The key idea is that variables get their names from the investigation, not from the object itself. Temperature might be an independent variable in one experiment and a controlled variable in another.
From Question To Method
When OCR asks for experimental design, it is usually testing whether the method matches the biological problem. A practical plan should make the relationship between the independent and dependent variables observable.
Use this route:
- State the biological question in practical terms.
- Choose a suitable range or set of values for the independent variable.
- Decide exactly how the dependent variable will be measured.
- Select apparatus and techniques that can make those measurements.
- Build in controls so other variables do not explain the outcome.
Planning From A Practical Question
Question: A student wants to investigate the effect of pH on the rate of an enzyme-controlled reaction.
Step 1: The independent variable is pH, so the method needs several pH values, not just "acidic" and "alkaline".
Step 2: The dependent variable is rate of reaction. A suitable measure might be time taken for a fixed amount of product to form, volume of gas produced per minute, or change in absorbance per minute, depending on the enzyme reaction.
Step 3: The apparatus must match the measurement. If gas is produced, a gas syringe is more suitable than simply watching for bubbles because it gives quantitative data.
Step 4: Temperature, enzyme concentration, substrate concentration and volumes should be kept constant because each could also affect rate.
Notice the reasoning: the apparatus is not chosen because it sounds scientific. It is chosen because it can measure the dependent variable with enough detail to test the expected outcome.
For "design" or "suggest a method" questions, link each method choice to the data it would produce. A named piece of apparatus is stronger when you also say what it measures.
Now use that link between measurement and apparatus in a familiar practical context.
From Question To Method Continued
For a written plan, a simple variable table can stop the method becoming muddled:
| Variable type | Example in the pondweed investigation | Why it matters |
|---|---|---|
| Independent | Distance between lamp and pondweed | Changes light intensity. |
| Dependent | Oxygen volume produced per minute | Gives evidence of photosynthesis rate. |
| Controlled | Temperature | Temperature also affects enzyme-controlled reactions in photosynthesis. |
Controlled Variables
A controlled variable is a factor that could affect the dependent variable but is not the factor being investigated. It should be kept constant, standardised, or accounted for where appropriate.
Controlled Variable
A variable, other than the independent variable, that could affect the dependent variable and should be kept constant or controlled so the comparison is valid.
Controlled variables are not a random list of everything in the room. They are chosen because there is a biological reason they might affect the outcome.
For example, suppose a student investigates the effect of ethanol concentration on beetroot membrane permeability by measuring the colour intensity of pigment released from beetroot cylinders.
| Variable to control | How to control it | Why it affects the outcome |
|---|---|---|
| Size of beetroot cylinders | Use a cork borer and cut cylinders to the same length | Larger pieces have more membrane and pigment. |
| Time in ethanol | Place all cylinders in ethanol for the same duration | Longer time may release more pigment. |
| Temperature | Use the same room temperature or a thermostatically controlled water bath if suitable | Temperature can affect membrane permeability. |
| Volume of ethanol | Use the same measured volume in each tube | Different volumes would dilute the pigment differently. |
The strongest answers name the variable, describe how it is controlled, and explain why it matters. "Keep the beetroot the same" is weaker than "use beetroot cylinders with the same diameter and length so each sample has a similar surface area and pigment content."
A controlled variable earns more credit when it is linked to the dependent variable: control it because otherwise it could change the result.
The next check asks for exactly that three-part answer: name, method of control, and reason.
Controlled Variables Continued
Sometimes a variable cannot be controlled completely. In a field investigation, soil moisture, shade or organism distribution may vary naturally. A good plan then accounts for the variable, for example by random sampling, stratified sampling, using a larger sample size, or recording the factor so it can be considered when interpreting the results. The planning skill is to recognise the problem before the data are collected.
Appropriate Apparatus And Techniques
Apparatus and techniques are appropriate when they fit the expected outcome, the measurement needed, and the biological context.
Think of apparatus choice as a match between the data needed and the tool's purpose:
| Expected data needed | More appropriate choice | Less appropriate choice |
|---|---|---|
| Small volumes of liquid | Micropipette or graduated pipette | Unmarked dropping pipette |
| Continuous change in absorbance | Colorimeter | Judging colour by eye |
| Volume of gas produced | Gas syringe | Counting irregular bubbles only |
| Movement of water in a potometer | Capillary tube with scale | Unmarked tubing |
| Temperature treatment | Controlled chamber or suitable water bath, depending on organism and setup | Leaving samples at "warm" or "cold" temperatures without measurement |
The same apparatus may be appropriate in one context and inappropriate in another. A water bath may suit test tubes containing enzyme solutions, but it would not be suitable for maintaining temperature around the leaves of a land plant in a potometer. The biological setup matters.
When selecting techniques, ask:
- Will the technique measure the dependent variable directly enough?
- Is the range of independent-variable values wide enough to show a pattern?
- Are the intervals close enough to detect a change?
- Can the method be repeated under the same conditions?
- Is the method safe and realistic for the organism or material used?
This is where scientific knowledge supports planning. If light intensity is being investigated, temperature may need controlling because lamps can heat the sample. If enzyme activity is being investigated, pH and temperature often need controlling because they affect enzyme shape and activity. If transpiration is being investigated, humidity, air movement and leaf area matter because they affect water loss.
For example, a gas syringe is usually more appropriate than counting bubbles when measuring gas production because it measures gas volume quantitatively. Bubble counts are less secure because bubbles can vary in size, so the count may not represent the actual gas volume produced per unit time.
An appropriate method does not need to be complicated. It needs to be matched to the expected outcome and controlled well enough that the results could be interpreted.
Evaluating Method Appropriateness
OCR can ask whether an experimental method is appropriate to meet the expected outcomes. This is a planning judgement. You are not just saying whether the method is neat; you are deciding whether it could answer the question.
Use four checks:
| Check | Question to ask | Example of a problem |
|---|---|---|
| Variable match | Does the method vary the intended independent variable? | A "temperature" investigation where samples are only labelled warm and cold. |
| Measurement match | Does the method measure the dependent variable clearly? | "Observe colour" when a colorimeter would give quantitative absorbance. |
| Control match | Are important confounding variables controlled or accounted for? | Different-sized beetroot pieces in a membrane-permeability test. |
| Outcome match | Would the results show whether the expected outcome happened? | Only one treatment value, so no pattern or comparison is possible. |
Evaluating A Weak Method
A student wants to investigate whether caffeine concentration affects Daphnia heart rate. The student plans to place one Daphnia in a caffeine solution for 30 seconds, count heartbeats for 10 seconds, and compare this with a different Daphnia in distilled water.
Appropriate features: the method has a clear independent variable, caffeine concentration, and a measurable dependent variable, heart rate.
Problems: only one caffeine concentration is tested, so the method cannot show a concentration pattern. Different Daphnia are used, so natural variation between organisms may affect the comparison. The concentration and exposure time should be chosen to minimise harm, the exposure time should be standardised, and repeats would increase confidence in the comparison.
Judgement: the method could give a rough comparison, but it is not fully appropriate for testing the effect of caffeine concentration because the range and biological variation are poorly controlled.
Be careful with the word "valid". A valid comparison is one where the method tests what it claims to test. That usually means the independent variable is changed deliberately, the dependent variable is measured appropriately, and other important variables are controlled.
Evaluating Method Appropriateness Continued
When evaluating a method, give a balanced judgement where possible: identify what is appropriate, then identify the method feature that prevents the expected outcome being tested well.
A Planning Answer Frame
For unfamiliar practical contexts, use a short answer frame. It keeps your response focused on OCR's planning skill.
- The independent variable is...
- The dependent variable should be measured by...
- The apparatus or technique is suitable because...
- The controlled variables include...
- These are controlled by...
- This makes the method appropriate because the results would show...
Here is the same frame applied to a new context:
Planning A Valid Comparison
Context: Investigate whether exercise intensity affects breathing rate.
Independent variable: exercise intensity, such as walking, jogging and running at defined speeds.
Dependent variable: breathing rate, measured as breaths per minute immediately after each exercise period.
Apparatus or technique: a stopwatch can time one minute accurately; a treadmill or marked route can standardise intensity more clearly than vague descriptions such as "hard exercise".
Controlled variables: duration of exercise, rest time between trials, same participant or matched participants, room temperature, and how breathing rate is counted.
Appropriateness judgement: the method would be more appropriate if each intensity is repeated after a standard rest period because the results could then be compared under similar conditions.
This kind of answer is flexible. In one context the important controlled variable might be temperature; in another it might be leaf area, pH, age of organisms, light intensity or volume. The rule is always the same: control the factors that could affect the dependent variable and therefore confuse the effect of the independent variable.
A Planning Answer Frame Continued
A final diagnostic check: explain your method to yourself in one sentence. If the sentence cannot name what is changed, what is measured, what is controlled, and why the method would show the expected outcome, the plan is not ready yet.