Planning an Investigation (Cambridge (CIE) 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

• Examiner reports show that students lose marks by:

  • listing apparatus without describing how it is used

  • not giving enough detail in their method

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 that is precise enough for 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, improving precision

  • 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, e.g. in a diagram within an exam question

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

      • Pick two labelled points next to each other on the scale (e.g. 20 and 30); determine the difference between the two numbers, i.e. 30 - 20 = 10

      • Count the number of spaces between those numbers (e.g. 5)

      • Divide the difference by the number of spaces: 10 ÷ 5 = 2

      • Each small division = 2

  • how to read a scale to the correct level of precision

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

    • E.g. 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

      • ensure that you are at eye level with the liquid to avoid parallax error

      • read from the bottom of the meniscus

  • how to record readings from digital instruments

    • Write down every digit shown on the display, e.g. do not write 5.0 g as 5 g; the trailing zero (.0) shows the precision 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 stop-clock shows minutes and seconds, convert values to match the units in the header, e.g. seconds, before entering them into a table

• 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 reliable"

• 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

  • E.g. 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