Practical and Experimental Skills, Including Internal Assessment
This domain covers the practical programme and the Internal Assessment (IA). The IA is an individual scientific investigation worth 20% of the final grade, assessed against criteria for research design, data collection and processing, analysis, evaluation and communication. Practical skills are also examined in Paper 3 Section A, where you interpret data and suggest improvements.
Core rules and concepts
- Research question: focused, testable and linked to theory. Identify independent and dependent variables, and predict a relationship from background knowledge.
- Method: control variables that could affect the result; choose apparatus with appropriate precision; repeat measurements (at least three, ideally five) and include a control where relevant.
- Uncertainty: record the uncertainty of each instrument and justify it. For addition and subtraction, add absolute uncertainties; for multiplication and division, add percentage uncertainties. Quote final answers to the appropriate significant figures.
- Data processing: tables with units and uncertainties; graphs with labelled axes, units, line of best fit and error bars; gradients with units.
- Evaluation: distinguish random errors (scatter, affect precision) from systematic errors (shift, affect accuracy); suggest realistic, specific improvements.
Worked example
A student finds the density of a metal: mass = 5.00 ± 0.05 g, volume = 2.00 ± 0.02 cm³. Density = 5.00/2.00 = 2.50 g cm⁻³. Percentage uncertainty in mass = (0.05/5.00) × 100 = 1.0%; in volume = (0.02/2.00) × 100 = 1.0%. Total percentage uncertainty = 2.0%, so the absolute uncertainty is 0.05 g cm⁻³. Report density as 2.50 ± 0.05 g cm⁻³. The reasoning earns marks because it shows the propagation rule and the correct significant figures.
Traps that cost marks
Do not write “human error” as an evaluation-name the specific error and its effect. Do not quote more significant figures than the least precise measurement allows. Do not confuse accuracy with precision. Do not omit units on graph axes. Do not claim a trend without a quantitative comparison.