Practical questions always ask how good your readings are: how precisely you read the instrument, which result does not fit, whether two values agree, and what precautions you took. Practise each one, with drawn instruments and worked solutions.
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Record each reading to the smallest division of the instrument: 0.1 cm on a millimetre ruler, every digit on a digital meter.
Spot the reading that does not fit, leave it out or repeat it, and average the rest.
Time 10 or 20 swings and divide, so reaction time matters less.
Values within about 10% are usually taken to agree within the limits of experimental accuracy. Justify your answer with the numbers.
A specific action for this experiment, such as reading at eye level to avoid parallax. General advice earns nothing.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.