# Measurements and Uncertainties

Accuracy: An experiment is accurate if the quantity being measured has a value that’s very close
to the commonly accepted or true value. For example an experimental value for the acceleration
of free fall of 9.78 m s-2 is much more accurate than an experimental value of 9.05 m s-2
.
> Precision: The results of an experiment are precise if they are close together and have a small
range. The smaller the range of the repeated values, the higher the precision. The term precision
is linked to the spread of the data or the percentage uncertainty in a measurement. A precise
experiment has a smaller spread in the data or the smaller the uncertainty. Hence, an
experiment with an acceleration of free fall of (9.05 ± 0.05) ms-2
is much more precise than an
experiment with (9.78 ± 1.20) ms-2
; but the latter is much more accurate.

Systematic Error: A systematic error is an error that does not happen by chance but instead is
introduced by an inaccuracy in the apparatus or its use by the person conducting the
investigation.
o Zero Error: an error that occurs when the apparatus shows a non-zero value when it
should be registering a value of exactly zero.
o Parallax Error: an error produced whenever a scale, gauge, or pointer is observed
wrongly during scientific experimentation due to position of viewing and perception.
> Random Error: A random error is an experimental error caused by unknown or unpredictable
changes to the apparatus or conditions.
> Random errors can be reduced, by improving techniques, making multiple measurements or
using instruments with a higher degree of precision. This increases precision.
> Systematic errors can be reduced or totally eliminated by the use of better techniques and
instruments. This increases the accuracy of the experiment.
> Resolution (precision): The smallest change in a quantity that an instrument can measure. The
higher the resolution, the higher the precision in repeat readings.
>If a measurement or test is reliable, it gives consistent results each time the activity is repeated.
A reliable experiment has consistent results for the same measurement. Experiments are
unreliable if repeated measurements give different results or if the scatter of measurements on a
line graph is large (or so great that establishing a line of best fit is difficult).