The digit preference bias may occur if study subjects or observers (interviewers) prefer differently certain numbers leading to differences in rounding up measurements. The observer (interviewer) bias results from systemic differences in repeated measurements from the same observer (intraobserver) or inconsistencies attributed to differences in the way applied by different observers (interobserver). The instrumental bias is due to faulty calibration, inaccurate measuring instrument, contaminated reagents, incorrect dilution, ..etc.