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Planning an investigation: the 6-mark method

Edexcel IAL Biology · WBI13 Practical Skills in Biology I

Unit 3 usually has a longer 'devise' or 'describe how you would' question set in a new context. A good plan is the same shape whatever the topic.

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A structure that works every time

1. What you will change and the actual values (at least five values of the IV, with a range, for example 0, 0.5, 1.0, 1.5, 2.0%).

2. How to change it (dilute a stock solution; set water baths).

3. What you will measure (DV) and how, with apparatus and units.

4. Controlled variables relevant to this context and how each is controlled.

5. Repeats (at least three per value) and calculation of means and standard deviations.

6. A control where relevant.

7. How the data will be used: table, graph (or bar chart) and, where appropriate, a named statistical test.

8. A brief safety point with its reason.

The January 2025 report: 'It's the same for all of them regardless of what the topic is.'

Be specific

Preliminary work

A short trial run before the real experiment can decide a suitable range of the IV, a suitable time to run each test, the concentration of a reagent, or whether the DV changes enough to measure. The January 2024 Unit 6 report encourages thinking about how volumes, concentrations and times in a method were chosen.

Worked example

Outline how you would investigate the effect of sucrose concentration on the growth of pollen tubes.

  1. IV: sucrose solutions 0, 5, 10, 15, 20% made by diluting a stock.
  2. Add a drop of each to a cavity slide, sprinkle pollen from the same flower species, cover slip.
  3. Keep at the same temperature (25 °C incubator) for the same time (for example 1 hour).
  4. DV: measure the length of 10 pollen tubes per slide with a calibrated eyepiece graticule; or count the percentage of grains germinated.
  5. Three slides per concentration; calculate means and SDs; plot mean length against concentration; use a correlation test.

Answer: A specific, repeatable plan: five concentrations, same species, temperature and time, measure tube length with a graticule, repeats, means and SDs, graph and correlation test.

Key idea: Plan: IV values and how made; DV and how measured; relevant controlled variables and how; repeats with means and SDs; control; how data are used; safety. Be specific and standardise biological material.

Check yourself

  1. How many different values of the independent variable should a plan usually include to show a trend?
    • At least five
    • One
    • Two
    • As many as possible with no repeats
    Show answer

    At least five. Five values give enough points to see a pattern.

  2. A plan says 'measure the size of each fish'. Why does this not gain credit?
    • Fish cannot be measured
    • Size is a controlled variable
    • It does not say which aspect of size or how it is measured
    • It should be measured only once
    Show answer

    It does not say which aspect of size or how it is measured. Say length from snout to tail with a ruler, or mass on a balance.

  3. How should a range of concentrations be made from a 20% stock sucrose solution?
    • Dilute measured volumes of stock with distilled water
    • Add different amounts of solid sucrose by eye
    • Heat the stock to evaporate water
    • Use tap water of different temperatures
    Show answer

    Dilute measured volumes of stock with distilled water. Serial or simple dilution with measured volumes.

  4. What is a purpose of preliminary work?
    • To collect the final results
    • To avoid doing a risk assessment
    • To make the results more significant
    • To find a suitable range of the independent variable or time for each test
    Show answer

    To find a suitable range of the independent variable or time for each test. Trials guide choices such as range, time and concentration.

  5. Which describes how data from five concentrations with three repeats each should be used?
    • Calculate a single total
    • Calculate means and SDs, plot mean against concentration and use a correlation test
    • Plot each repeat as a bar chart with no mean
    • Only quote the highest value
    Show answer

    Calculate means and SDs, plot mean against concentration and use a correlation test. Means, spread, graph and test.

  6. When comparing heart mass with fish size, which is a suitable standardisation?
    • Use hearts from different species
    • Weigh some hearts wet and some dry
    • Measure only the largest fish
    • Remove blood and trim tissue the same way before weighing each heart
    Show answer

    Remove blood and trim tissue the same way before weighing each heart. The June 2024 mark scheme credited standardising heart preparation.

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Last updated 8 November 2026. Spotted a mistake? Tell us.