IGCSE Biology Practical Skills 2: Planning, Variables and Evaluation

Cambridge IGCSE Biology Papers 5 and 6 use the same AO3 planning and evaluation skills. A defensible plan changes one defined factor, measures a relevant response, controls alternatives, manages risk and states how evidence will answer the question. Evaluation then diagnoses what the evidence can and cannot support.

Turn a question into testable variables

The independent variable is deliberately changed. The dependent variable is observed or measured in response. Control variables are factors that could affect the dependent variable and are kept constant to make the comparison valid.

Name variables operationally. "Temperature" is incomplete as a method; state the selected temperatures, how they are produced and how actual reaction temperature is checked. "Enzyme activity" needs a measurable endpoint such as time for starch to disappear or gas volume produced per minute.

A hypothesis predicts a relationship and may include biological reasoning. A prediction describes the expected result in the proposed setup. Do not write a conclusion before evidence exists.

Choose a useful range and intervals

Use enough independent-variable values to reveal a relationship, curve or optimum. Five values are often stronger than a treated and untreated pair, although the context determines what is feasible.

The range should be safe and biologically informative. A pilot study can identify a useful range, suitable intervals and an endpoint that is neither instantaneous nor too slow.

Intervals need not always be equal, but closer values can be placed near a suspected transition or optimum. Avoid claiming the true optimum when only a narrow or widely spaced range was tested.

Control alternative explanations

For every important control variable, state how it is held constant and why. In an enzyme-temperature investigation, control enzyme and substrate concentrations, their volumes, pH and endpoint method because each could change measured time.

Standardise biological material by species, source, age, initial size or surface area where relevant. Biological specimens are not identical, so independent replicates may be needed even when dimensions are controlled.

A control experiment removes the proposed causal factor while keeping other conditions similar. Boiled seeds can provide a non-living comparison for respiration, and distilled water may provide a negative food-test control. A control variable and a control experiment are different ideas.

Write a reproducible method

Specify apparatus, quantities, concentrations, sequence, timing and endpoint. Another student should be able to perform the investigation without inventing missing decisions.