What is a 'zero error' and how can I correct for it?

A zero error occurs when an instrument gives a non-zero reading when it should actually read zero. For instance, if a weighing scale shows a reading of 50 grams when nothing is on it, it has a zero error of 50 grams. To correct for this, you can either recalibrate the instrument to read zero or subtract the zero error from all subsequent measurements.