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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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