What causes positive and negative deviations when weighing analytical samples?

Positive deviations are when the measured mass is more than the true mass of the sample and negative deviations are when the measured mass is less than the true mass of the sample. What steps causes this type of errors in analytical measurements?

• In addition to the answer by @M. Farooq , there is always random electronic measurement error (aka "noise") in electronic devices, e.g., electronic analytical balances. So there are both random measurement errors and systematic errors. You might also check out Appendix G in the 2012 QUAM: eurachem.org/images/stories/Guides/pdf/QUAM2012_P1.pdf . Hope this helps! – Ed V Jul 19 '19 at 13:59