Systematic errors
are errors that always have the same magnitude and sign, resulting in a bias of the measured values from the true value. An example would be a ruler missing the first 1 mm of its length – it will consistently give lengths that are 1 mm too short. Systematic errors affect the accuracy of the final result.
Random errors
will have different magnitudes and signs, and result in a spread or dispersion of the measured values from the true value. An example would be any electronic measuring device – random electrical noise within its electronic components will cause the reading to fluctuate, even if the signal it is measuring is completely constant. Random errors affect the precision of the final result; they may also affect accuracy if the number of replicates used is too small.
Gross errors
are errors that are so serious (i.e. large in magnitude) that they cannot be attributed to either systematic or random errors associated with the sample, instrument, or procedure. An example would be writing down a value of 100 when the reading was actually 1.00. If included in calculations, gross errors will tend to affect both accuracy and precision.