Uncertainty and errors · Grade 9
No measurement is perfect. Uncertainty tells you how far from the true value a result might be, and spotting errors helps you improve an experiment. Repeating readings, using better instruments and drawing careful graphs all reduce uncertainty.
Key facts
- Random errors scatter readings either side of the true value; repeat and average to reduce them.
- Systematic errors shift every reading the same way (zero error, parallax); repeating does not remove them.
- Accurate: close to the true value. Precise: readings close to each other.
- Uncertainty in a mean ≈ half the range of the repeat readings.
- Percentage uncertainty = uncertainty ÷ value × 100%.
- Adding or subtracting: add absolute uncertainties. Multiplying or dividing: add percentage uncertainties. Raising to a power n: multiply the percentage uncertainty by n.