Practical skills — Cambridge IGCSE Biology

Revision notes

7 short notes, in the order of the specification. Each one in short:

1. Planning an investigation: variables, controls, range and repeats

   Practical

   Planning an investigation means stating exactly what you change, what you measure, what you keep the same and how. The independent variable is changed, usually over at least five evenly spaced values; the dependent variable is measured with named apparatus. Keep controlled variables constant, include a control experiment, repeat each test at least three times and calculate a mean.

2. Recording results: tables, units and the solidus

   A Cambridge results table is ruled, with the independent variable in the first column and each heading giving the quantity and unit separated by a solidus, such as time / s. Units go only in the heading, never in the body. Record data to the precision of the instrument, to the same decimal places in each column.

3. Drawing graphs and charts the Cambridge way

   A line graph is used when the independent variable is continuous, such as temperature, while a bar chart, with bars that do not touch, is used for categories. Put the independent variable on the x-axis, label axes with quantity and unit, use more than half the grid, plot small crosses and draw a single thin best-fit line or curve.

4. Biological drawings and measuring specimens

   Practical

   A biological drawing must be large, with clear, unbroken pencil lines, no shading or colour, and ruled label lines that touch the feature without arrowheads. Measure sizes in millimetres with a ruler. Magnification equals image size divided by actual size, both in the same unit, and has no unit; it is written with a × sign, such as ×3.

5. Evaluating experiments: errors, anomalies and improvements

   Evaluating an experiment means judging how good the method and data are, then suggesting specific improvements. Identify any anomalous result, a result that does not fit the pattern, and leave it out of the mean. Name sources of error and exact fixes, such as using a syringe instead of a beaker. Human error or being more accurate scores nothing.

6. Indicators and tests: limewater, hydrogencarbonate, universal indicator and litmus

   Practical

   Indicators are substances that change colour to show a chemical change, and you must give the colour before and after. Limewater turns from colourless to milky with carbon dioxide. Hydrogencarbonate indicator is red at the level in air, yellow when carbon dioxide increases and purple when it decreases. Universal indicator gives the pH number; litmus shows only acid or alkali.

7. Sampling: random sampling, sample size and avoiding bias

   Random sampling is choosing samples so that every individual or position has an equal chance of being chosen, for example using random number coordinates on a grid of tape measures. It avoids bias, which can happen when you choose positions yourself or throw a quadrat. A larger sample size, such as 10 to 20 quadrats, makes results more representative.