Relating the Error to Possible Causes

Random errors (imprecision) and systematic errors (bias) have different causes. Systematic errors are more common and usually easier to investigate.

• Systematic error is evidenced by a change in the mean of the control values. The change in the mean may be gradual and demonstrated as a trend or it may be abrupt and demonstrated as a shift. Causes of systematic error include change in reagent lot, change in calibrator lot, wrong calibrator values, improperly prepared reagents, deterioration of reagents, deterioration of calibrator, inadequate storage of reagents or calibrators, change in sample or reagent volumes due to pipettor misadjustments or misalignment, change in temperature of incubators and reaction blocks, deterioration of a photometric light source, and change in procedure from one operator to another.
• Random error is any deviation away from an expected result. For QC results, any positive or negative deviation away from the calculated mean is defined as random error. Random error can be acceptable (or expected) as defined by the laboratories acceptable range or unacceptable (unexpected), which is any data point outside the expected population of data (for example, a data point outside the ±3SD limit). Random errors can be caused by bubbles in reagents and reagent lines, inadequately mixed reagents, unstable temperature and incubation, unstable electrical supply, and individual operator variation in pipetting, timing, and so on.

  Erratic performance due to occasional air bubbles in sample cups or syringes or defective unit-test devices are a different kind of random error, often called "flyers." Flyers are not actually caused by a change in the imprecision of the method, but rather represent an occasional disaster. It is very difficult to catch flyers using quality control. Patient replicate determinations are a better way of detecting these types of events.