In a screw gauge experiment to measure the diameter of a chalk piece, three readings are recorded: d1 = 1.002 cm, d2 = 1.004 cm, d3 = 1.006 cm. Which of the following statements is/are correct?

Mean absolute error in radius is 0.0013 cm
Mean absolute error in diameter is 0.001 cm
The error is 0 cm
Percentage error in the measurement of diameter is 1.3%

Correct answer: Mean absolute error in diameter is 0.001 cm

Solution

Mean absolute error in diameter = (0.002 + 0 + 0.002)/3 = 0.00133 cm, which rounds to 0.001 cm (1 sig fig). Mean absolute error in radius = 0.00067 cm, not 0.0013 cm. Percentage error = 0.00133/1.004 * 100 = 0.13%, not 1.3%. Error is not zero since readings differ.

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