Precision describes agreement among repeats under stated conditions. Accuracy concerns agreement with a suitable reference. Close repeats can all be biased. Resolution is the smallest readable marked step, not a guarantee of accuracy. Do not add unsupported digits. Safety: Use a light closed book and intact ruler on a stable surface; no medical or chemical measurements. Stop: End after five readings and two practice values. Do not calibrate equipment, weigh substances or add unsupported decimal places.

Materials: Millimetre-marked ruler and one closed book; Paper and pencil 1. Choose a straight book edge shorter than the ruler. Align zero with one end; look directly above the other. Record centimetres to the nearest millimetre. Lift and reset for five readings; note the smallest division. 2. Compare consistency. You have no trusted book-length reference, so repeats alone cannot establish accuracy. Check for a damaged zero mark or angled viewing. 3. Analyse this illustrative reference case: assigned mass 100.0 g; balance A reads 105.0, 105.1, 105.0 g; B reads 99.9, 100.0, 100.1 g. Describe consistency and reference agreement. No real balance was calibrated. 4. Round these illustrative values: 12.36 cm to three significant figures; 0.00476 m to two. Keep the requested digits from the first non-zero digit, then inspect the next digit for rounding. Leading zeros do not count. A next digit 0–4 leaves the retained digit unchanged; 5–9 rounds it up. 5. Write each rounded result in standard form: a number at least 1 and less than 10 multiplied by a power of ten. For measuring about 5 mm, choose 1 mm or 1 cm ruler divisions and explain the resolution difference without promising accuracy. Done: Five actual length readings, a reference-case comparison, two rounded standard-form values and an instrument-choice reason are recorded. Proof: Keep raw readings and calculations. Note ruler divisions, alignment help and no book-length reference. Stop: End after five readings and two practice values. Do not calibrate equipment, weigh substances or add unsupported decimal places.