Planning an Investigation (AQA GCSE Biology): Revision Note

How to write a method

• A planning question asks you to describe how you would carry out a specific investigation, using a sequence of instructions that someone else could follow

AQA required practicals

• You must carry out ten required practicals, and questions based on them make up at least 15% of the marks:

  1. microscopy

  2. microbiology (antiseptics or antibiotics)

  3. osmosis

  4. food tests

  5. enzymes (effect of pH on amylase)

  6. photosynthesis (light intensity)

  7. reaction time

  8. plant responses (light or gravity)

  9. field investigations

  10. decay (temperature and milk)

Writing a method

• Write your method as a set of steps, in the order you would do them

• A good method includes:

  • how to set up and use the apparatus

  • the quantities to use, with units, e.g. 25 cm³ of water

  • how to change the independent variable, and the range of values to use

  • how and when to measure the dependent variable, and which apparatus to use

  • which variables to keep constant, and how

  • how many times to repeat each reading

• Examiner reports show that students lose marks by:

  • listing apparatus without describing how it is used

  • not giving enough detail in their method

• In a 6-mark Plan question, your method must lead to a valid outcome, so include enough repeats or participants, the control variables, and how you will process the results, e.g. calculate a mean

• Adapt the required practical you did in class to the exact question

Choosing & justifying apparatus

• When describing apparatus choices in practical questions, you should name the specific piece of apparatus

  • E.g. a measuring cylinder, not "something to measure volume"

• Common apparatus items for measuring different variables include:

  • volume: measuring cylinder, gas syringe or pipette

  • mass: electronic balance

  • length / short distances: ruler

  • time: stopwatch

  • temperature: thermometer

• Choose apparatus with a resolution that suits the variable being measured, e.g. for measuring 2 cm³, a 5 cm³ syringe will be more appropriate than a 100 cm³ measuring cylinder

• When a question asks you to justify your choice, give a reason linked to the experiment, e.g.:

  • use a gas syringe to measure oxygen production rather than counting bubbles; this is because bubbles vary in size, while a gas syringe measures the volume of gas produced, so the result does not depend on bubble size

  • use a temperature-controlled water-bath rather than a beaker of hot water; this is because a beaker of hot water cools down over time, while a water-bath maintains a constant temperature, so this control variable won't affect the results

Taking readings to the correct resolution

• When it comes to reading results and recording measurements correctly, you need to consider how to determine the size of a single small division on a scale

  • Sometimes you may not know the size of a single small division on a scale, for example, in a diagram within an exam question

• In order to determine the size of a single division you need to:

  • find two labelled values on the scale, e.g. 20 and 30

  • count the small divisions between them, e.g. 5

  • divide the difference by the number of divisions, e.g. 10 ÷ 5 = 2 units per division

• How to read a scale to the resolution of the instrument:

  • Once you know what one small division is worth, you can read to the nearest line or half-division

    • For example, on a thermometer marked every 1 °C, you can read to the nearest 0.5 °C

    • So for a reading that falls in between 68 and 69 °C, you could record 68.5 °C, but it would not be reasonable to record 68.3 °C

• How to take readings with a ruler, and from a liquid:

  • Always take ruler measurements in mm, e.g. 42 mm, rather than in cm, when measuring for magnification calculations

  • When reading liquid volumes, read the bottom of the meniscus at eye level to avoid parallax error

• How to record readings from digital instruments:

  • Write down every digit shown on the display

    • For example, do not write 5.0 g as 5 g

    • The trailing zero (.0) shows the resolution of the balance

• How to record values in a results table:

  • All readings from the same instrument down a column must have the same number of decimal places

  • Put units in the header, never in the body of the table

  • If a digital stopwatch shows minutes and seconds, convert values to match the units in the header before entering them into a table

    • Or create an extra column to do the calculation

• Examiner reports show that students lose marks by:

  • recording readings to different numbers of decimal places

  • not giving a unit

Repeats, sample size & random sampling

Repeating an experiment

• Repeating an experiment lets you spot anomalous results and calculate a mean

  • Carrying out at least three trials in total for each value of the independent variable allows anomalous results to be spotted

    • These anomalies can then be excluded from the mean

    • The more repeats are carried out, the less impact each individual result has on the mean

  • Calculating a mean reduces the impact of random error on the results

Sampling

• Testing a large sample size, e.g. 100 leaves or 50 seeds, increases the likelihood that your data is representative of the whole population

• Choose samples at random, so that you do not pick unusual ones without meaning to

  • Give a reason that is specific to the experiment, not a general phrase such as "to make it more accurate"

  • In the field investigation required practical (RP9), place quadrats at random, e.g. using random coordinates, to sample a population

• Examiner reports show that students lose marks by:

  • suggesting only one repeat, instead of at least two more

  • giving general reasons, e.g. "to improve reliability or accuracy", instead of a reason linked to the experiment

Making a prediction

• A prediction says what you expect to happen before you do the experiment

  • A good prediction gives a reason, using the biology you know

  • For example, as temperature increases up to 40 °C, the rate of enzyme activity will increase, because molecules will have more kinetic energy

• AQA defines a predication as:

A statement suggesting what will happen in the future, based on observation, experience or a hypothesis

• A hypothesis is a proposal intended to explain certain facts or observations

• A prediction is not a conclusion

  • A conclusion describes what your results show after the end of an experiment

  • For example, the results show that as temperature increased from 20 °C to 40 °C, the rate of enzyme activity increased from 1.2 to 4.8 cm³/min, because higher kinetic energy increased the rate of successful collisions