IB Physics Measurement & Uncertainties: FAQ

It's Topic 1 of the IB Physics guide (first exams 2025), covering SI units, significant figures, uncertainty types, and error propagation. It's rarely a standalone exam question — instead it's embedded in Paper 1 data questions, Paper 2 calculations, and every Internal Assessment, where it affects your data-processing marks directly.

Random errors scatter results above and below the true value — caused by reaction time or reading a scale — and you reduce them by repeating trials and averaging. Systematic errors shift every reading the same direction, like a zero-error on a mass balance, and repeating the experiment won't fix them; you have to correct the method.

Percentage uncertainty is (absolute uncertainty ÷ measured value) × 100. For multiplication or division, add the percentage uncertainties of each quantity. For addition or subtraction, add the absolute uncertainties instead. Convert back to an absolute uncertainty at the end and round it to match your final answer's significant figures.

Draw a line of best fit, then the steepest and shallowest lines that still pass through all the error bars — these are your max and min gradient lines. Calculate the gradient of each, and the uncertainty is half the difference between the max and min gradient values.

Match the significant figures in your final answer to the least precise piece of data you used — usually 2 or 3 sig figs in DP Physics. Writing 9.81234 m/s² when your data only supports 2 sig figs is an easy mark loss examiners flag constantly, even when your method is correct.

Quoting a final result without any uncertainty at all, or copying the equipment's stated precision without checking whether random scatter in your own repeated trials was actually larger. Examiners marking the Analysis criterion want to see you've engaged with your real data, not just parroted a ruler's ±0.5 mm.

The core content is identical — SL and HL students learn the same uncertainty rules, since Topic 1 sits in the shared syllabus core rather than the HL-only additional material. The difference is context: HL exams apply these skills to harder multi-step calculations across topics like fields and quantum physics.

No — there's no dedicated 'uncertainties' question on Paper 2. Instead, it appears as a sub-part of larger data-analysis questions, where you're asked to state an uncertainty, propagate it through a calculation, or comment on a percentage error compared to an accepted value.

You need to understand their precision and typical uncertainty (usually ±0.01 cm for a Vernier calliper, ±0.001 cm for a micrometer) rather than physically operate one in a written exam. Practical use matters far more for your IA and any school-based practical assessment.

The IB Physics data booklet doesn't list uncertainty-propagation formulas — you're expected to know the addition-in-quadrature-free rules (add absolute for +/-, add percentage for ×/÷) from your notes. Nothing about combining uncertainties appears in the booklet itself, which catches students out every year.

Both qualifications teach similar core skills — significant figures, percentage uncertainty, error bars — but the IB embeds it as an assessed skill across the IA and every exam paper, whereas A-Level tends to test it in more contained, separate questions. IB students end up applying it more often, in more varied contexts.

It's usually not a physics gap — it's a habit gap. Students often calculate the right numerical answer but forget to state, propagate, or round the uncertainty correctly, and mark schemes award those as separate marking points. It's a fixable, mechanical skill, not a conceptual one.

Past papers are the single best free resource — every Paper 1 and Paper 2 series includes uncertainty-based questions embedded across topics. Pair that with topic-specific worked examples so your child sees the propagation steps laid out, not just the final rounded answer.