in healthy individuals the blood test result remains virtually constant day by day
It is possible to use observations on healthy individuals for quality control of a routine lab
Analyze 5-10 healthy subjects at intervals and calculating mean and S.D. for RBC, WBC, Plt, etc.
On each occasion the mean should not vary by more than 2 S.D. and the S.D.s should remains constant
A significant difference in mean indicates a constant error e.g.: incorrect calibration
A random error will result in an increase in S.D. although the mean may be unaffected