Biochemical or physiological measurements are never completely accurate. Think of blood pressure machines and “white coat hypertension.”

Random errors (less serious) can occur when exposed and controlled specimens are analyzed randomly by different laboratories. Systematic errors occur when all specimens from the exposed group is analyzed in one lab and specimens from the controlled group are analyzed in another lab.

If the labs produce systematically different results when analyzing the same specimen, then the study has become biased.

Recall bias – differential recall of information by cases and controls.