Absolute uncertainty
The uncertainty in a measurement expressed as an absolute quantity in the same units as the measurement (e.g., ±0.5 m, ±2 g). It represents the range within which the true value likely lies. Absolute uncertainty is often estimated as the precision of the instrument or as the standard deviation of repeated measurements.
Formula
Absolute uncertainty = ±½ × smallest division (for single readings)
Real World
When a pharmacist at Boots weighs a 500 mg paracetamol tablet on a balance reading to ±0.5 mg, the absolute uncertainty of ±0.5 mg tells them the true mass lies between 499.5 mg and 500.5 mg.
Exam Focus
Always include the ± sign and correct units with your absolute uncertainty — omitting either loses marks on practical papers.
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