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How Science Works

Accuracy and Precision.

Accuracy tells you how close your measurements of
the variable are to the true value of the variable.

Precision tells you how precisely you can make
your measurements. Precision is shown by the number
of decimal places given for a measurement. It can be
expressed as a percentage of 100% minus the maximum error.

A measurement of 10 volts means that the value
is greater than 9·5 volts but less than 10·5 volts.
The difference between 9·5 volts and 10·5 volts is 1 volt
and so the maximum error is 1 volt in 10 volts, or 10%.
The precision of this measurement is
100% - 10% = 90%.

A measurement of 10·0 volts means that the value
is greater than 9·95 volts but less than 10·05 volts.
The difference between 9·95 volts and 10·05 volts is 0·1 volts
and so the maximum error is 0·1 volts in 10 volts, or 1%.
The precision of this measurement is
100% - 1% = 99%.

A measurement can be very precise but still be very inaccurate.
If the true value of the voltage is 20·0 volts then a measurement
of 10·0 volts has a 99% precision but only a 50% accuracy.

If data is to be used as evidence it must be accurate
and be measured as precisely as possible.

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