Understanding the sources of measurement errors
Errors in a measurement are deviations from the true (i.e. the actual) value. There are two generic sources of error: systematic and statistical.
Systematic errors are due primarily to defects in technique or limitations in equipment. For example, when you use the Pipettor, you assume that the value you withdraw is correct - but it may devitiate from the expected value for various reasions. The pipettor may be malfunctioning and need to be recalibrated.
Another important consideration is the inherrent acurracy of the device. Pipettors are normally calibrated to within a certain error range, for example + or - 0.5%. But if you use a pipettor designed to measure in the range of 0.2 to 1 ml to measure 0.02 mL your error may be much larger.
A related source of error is the failure to standardize the measuring devices. For example, a real or a virtual pH meter or spectrophotometer will drift with time, and must be recalibrated periodically to insure accurate measurement.
Alternatively, the error may arise through poor technique. For example, when using a vPipettor for small volumes, e.g. about 0.5-5 uL, it is important that excess liquid on the outside of the pipette tip be removed, without changing the volume of the liquid within the tip. This can be done by carefully wiping the tip with a Kimwipe ->
Similarly, it is not possible to measure accurately measure microliter volumes using a 10ml glasspipette. And when using a glass pipette, it is important to note whether it is a "to deliver" (TD) or "to contain" (TC) version.
As is usually the case, the more attentive you are to accuracy the more accurate your measurements will be.
Pipetting error #1
excess liquid on the outside of the pipette tip.
Pipetting error #2
using the wrong size pipette