Planning an Investigation (Edexcel IGCSE 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

  • 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

• In Edexcel papers, this is tested by "Design an investigation" questions worth 6 marks

• Use CORMS to plan your answer:

  • C: what you will change (the independent variable) and the range of values

  • O: the organisms you will use, all of the same species, age or size

  • R: repeat each value and calculate a mean

  • M: what you will measure (the dependent variable), how, and after what time

  • S: how you will keep two other variables the same

• Examiner reports show that students lose marks by:

  • writing "change the light intensity" without saying how it will be changed

  • not saying how the dependent variable will be measured

  • not giving a time for each measurement

Choosing & justifying apparatus

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

  • For example, 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 that is precise enough for the variable being measured

  • For example, 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

  • 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, giving a more accurate measurement

  • 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 precision

• 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

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

  • Repeating the experiment and calculating a mean makes the results more reliable

  • Repeats improve reliability, not accuracy

Sampling

• Testing a large sample size

  • For example, 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

  • Describe how the samples will be chosen at random, e.g. use random numbers to give coordinates on a grid

• Examiner reports show that students lose marks by:

  • saying that repeating an experiment makes it more accurate when it makes the results more reliable

  • suggesting repeats with different values of the independent variable, which does not improve reliability

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

• A prediction is not a conclusion: a conclusion describes what your results show after the end of an experiment

  • 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