Planning: purpose, hypothesis and variables
Three kinds of investigation
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The standard allows three types, and your choice determines what your method must describe:
- Fair test — you change one variable and measure its effect on another, controlling everything else.
- Pattern-seeking — you observe and measure without manipulating anything, looking for a relationship. Used where manipulation is impossible or unethical, such as in field ecology.
- Modelling — you use a physical or mathematical model to represent a biological system.
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Most school investigations are fair tests, and the rest of this page assumes one unless stated. The requirements for pattern-seeking are given alongside.
The purpose, written as a hypothesis
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Your purpose must be stated as a hypothesis that is linked to a biological concept or idea. Both parts are required.
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A hypothesis is a testable prediction, not a question and not a topic.
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A good hypothesis has three components:
- The independent variable and how it will change.
- The dependent variable and the predicted direction of its response.
- The biological reason — why you predict that.
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Compare these:
- Too vague: "To investigate photosynthesis in pondweed." This is a topic, not a prediction.
- A prediction, but unlinked: "As light intensity increases, the rate of oxygen production by pondweed will increase." Testable, but there is no biology in it.
- Achieved standard: "As light intensity increases, the rate of oxygen production by pondweed will increase, because light provides the energy absorbed by chlorophyll to drive the light-dependent reactions of photosynthesis, so more photons available means a faster rate — until another factor becomes limiting."
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The link to a biological idea is what the standard requires, and it is the most commonly omitted part. It also sets you up for Merit, because the explanation you give here is what your conclusion must return to.
Variables
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Independent variable (IV) — the one you change.
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Dependent variable (DV) — the one you measure, which responds to the IV.
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Controlled variables — everything else that could affect the DV, held constant so it cannot explain your results.
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For each, your method must state:
- IV: what it is, its range, and the number of levels.
- DV: what it is and how it is measured — the instrument, the units, and the precision.
- Controlled variables: what they are and how each is held constant. Naming them is not enough; you must say how.
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For a pattern-seeking investigation, the requirements differ: describe the data to be collected, the range of data or samples, and the other key factors you have considered — since you are not controlling variables, you must instead record the factors that might affect your measurements.
Choosing a valid range
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The range is the span from lowest to highest IV value. Choosing it well is a Merit requirement, and it is worth thinking about before you start.
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A valid range must:
- Span enough to produce a measurable change in the DV. Too narrow, and any pattern is buried in variation.
- Contain enough levels to show the shape of a relationship. Five or more is typical; two points cannot distinguish a straight line from a curve.
- Cover the biologically relevant part of the range — the interval where the response actually occurs. If you already expect a plateau, your range should include the rise, the transition and the plateau.
- Stay within survivable limits for living material. A range that kills your organism gives you no data about the process you were investigating.
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A pilot study — a quick trial run — is the reliable way to check your range before committing. Reporting that you ran one and adjusted your range as a result is strong evidence for Excellence, because it is a justified choice.
Controlling variables
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Your investigation is only valid if the IV is the only thing that changed. Every uncontrolled variable is an alternative explanation for your results.
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Typical controlled variables in biology, and how to control them:
- Temperature — water bath or controlled room, monitored with a thermometer, not assumed.
- pH — buffer solution.
- Light (when not the IV) — fixed distance from a fixed source, in a room with no other light.
- Organism variation — same species, similar age, size and mass, and ideally from the same source.
- Time — identical duration for every trial, timed rather than estimated.
- Volume and concentration of solutions — measured with appropriate glassware.
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Some variables cannot be controlled, particularly in the field. Where that is true, say so, explain why, and describe how you minimised or accounted for the effect. Acknowledging an uncontrolled variable honestly is worth far more than pretending it was controlled.
Safety and ethics
- Your procedures must follow Safety and Science: a Guidance Manual for New Zealand Schools, and investigations involving animals must comply with the Animal Welfare Act 1999.
- Practically, this means:
- Complete a risk assessment identifying hazards and the control measures for each.
- Investigations must not cause animals pain or distress. Observation and non-invasive measurement are acceptable; anything causing harm is not.
- Take account of cultural considerations where relevant, including obtaining appropriate permission before collecting native species or working at sites of significance.
Selective advantage
- Not applicable — this is a skills standard. There is no biological content to link to survival and reproduction. What is assessed is your investigative process and the quality of your reasoning about it.
Worked Example
Worked Example
A student plans to investigate how temperature affects the rate of an enzyme-catalysed reaction using catalase from potato.
Their first plan:
- Hypothesis: "Temperature affects catalase activity."
- Temperatures: 20 °C and 40 °C.
- Measurement: "Observe how much the mixture bubbles."
- Controlled variables: "Everything else kept the same."
Identify the weaknesses in this plan and rewrite it to Merit standard.
Answer:
Weaknesses.
- The hypothesis is not a prediction and has no biology in it. "Temperature affects catalase activity" states that there will be an effect but not its direction, so it cannot be tested — any result confirms it. It also gives no biological reason, which the standard requires.
- Two temperatures cannot establish a relationship. Two points can only produce a straight line, so the plan cannot detect the peak at an optimum temperature or the fall from denaturation — which is precisely what makes enzyme–temperature relationships interesting. The range is also too narrow to include denaturation.
- The DV is not quantitatively measured. "How much it bubbles" is a subjective judgement, not a measurement. There are no units and no instrument, so results cannot be processed, graphed or compared.
- Controlled variables are not identified. "Everything else kept the same" names nothing and describes no method of control, so a reader cannot judge whether the IV was the only thing that changed.
Rewritten plan.
Hypothesis. As temperature increases from 10 °C to 60 °C, the rate of oxygen production by catalase will increase to a maximum at around 40 °C and then decrease sharply.
This is because increasing temperature gives enzyme and substrate molecules more kinetic energy, so successful collisions between them become more frequent and the rate rises. Above the optimum, the increasing energy disrupts the hydrogen and ionic bonds holding the enzyme's tertiary structure, so the active site changes shape and can no longer bind the substrate. The enzyme is denatured and the rate falls.
Independent variable. Temperature, at 10, 20, 30, 40, 50 and 60 °C — six levels spanning the range from below to well above the expected optimum, so the rise, the peak and the fall can all be detected.
Dependent variable. The volume of oxygen produced in 60 seconds, collected in a gas syringe and read in cm3 to the nearest 0.5 cm3. This is quantitative, has units, and can be graphed.
Controlled variables, and how each is controlled:
- Temperature of the reaction — a thermostatically controlled water bath, with the reactants pre-equilibrated for 5 minutes before mixing and the temperature checked with a thermometer at the start and end of each trial.
- Substrate concentration — the same hydrogen peroxide solution throughout, 20 cm3 of 2% measured with a measuring cylinder.
- Enzyme quantity — potato discs of the same diameter and thickness, cut with a cork borer, the same number each trial, and all from the same potato so enzyme concentration is consistent.
- pH — the same buffer solution added to every trial.
- Time — exactly 60 seconds, measured with a stopwatch from the moment of mixing.
Reliability. Three trials at each temperature, with the mean calculated, so anomalies can be identified and the effect of random variation reduced.
Before starting, run a pilot at 10, 40 and 60 °C to confirm that measurable oxygen is produced at the extremes and that the gas syringe does not fill before 60 seconds at the optimum. If it does, reduce the substrate volume or the collection time and record why.
Safety. Hydrogen peroxide is an irritant — eye protection and gloves, and spills washed immediately. Care with hot water baths.