Practical skills — designing and evaluating investigations

The CCEA Double Award Science Practical Skills unit is assessed through a separate practical skills paper. However, practical skills knowledge is also tested within the B1, B2, C1, C2, P1 and P2 papers.

Types of variable

Independent variable (IV): the variable you change deliberately. You choose the values.

Dependent variable (DV): the variable you measure — it responds to changes in the IV.

Control variables: all other variables that could affect the DV, which you keep constant to make the test fair.

Example: Investigating the effect of temperature on the rate of enzyme activity.

• IV: temperature (°C)
• DV: rate of reaction (e.g. time taken for a colour change, or volume of product per minute)
• Control variables: pH, concentration of substrate, concentration of enzyme, volume of solution.

Fair tests

A fair test changes only the independent variable while keeping all other variables constant. If multiple variables change at once, you cannot determine which caused the change in the DV.

Hypotheses

A hypothesis is a testable prediction based on scientific understanding.

Format: "I predict that [IV change] will [cause DV change] because [scientific reasoning]."

Example: "I predict that increasing temperature will increase the rate of the reaction because higher temperatures give particles more kinetic energy, increasing the frequency and energy of collisions."

Repeats and reliability

Repeat readings are taken to check consistency and calculate a mean. Repeats improve reliability by identifying anomalous results (outliers) that may be due to error.

A reliable experiment gives consistent results when repeated under the same conditions.

A valid experiment actually measures what it intends to measure.

Risk assessment

Before practical work, potential hazards must be identified and control measures put in place.

• Identify the hazard (e.g. acid, hot water, sharp glass).
• State the risk (e.g. skin burns, cuts).
• State the control measure (e.g. wear gloves and goggles, do not heat above 60°C).

Common evaluation points for CCEA

When asked to evaluate an experiment, consider:

1. Reliability: were enough repeats taken? Were anomalies identified?
2. Validity: were control variables truly controlled?
3. Accuracy of measurement: which instrument was used? What is its precision? (e.g. measuring cylinder vs burette)
4. Improvements: suggest specific, realistic improvements (e.g. use a water bath to control temperature more accurately; take 5 repeats instead of 3).