2.
Measurement Process Characterization
2.3. Calibration


Purpose  The purpose of statistical control in the calibration process is to guarantee the 'goodness' of calibration results within predictable limits and to validate the statement of uncertainty of the result. Two types of control can be imposed on a calibration process that makes use of statistical designs:  
Shortterm standard deviation 
The shortterm standard deviation from each design is the basis for
controlling instrument precision. Because the measurements for a
single design are completed in a short time span, this standard
deviation estimates the basic precision of the instrument. Designs
should be chosen to have enough measurements so that the standard
deviation from the design has at least 3 degrees of freedom where
the degrees of freedom are (n  m + 1) with


Check standard 
Measurements on a check standard provide the mechanism for controlling
the bias and longterm variability of the calibration process. The
check standard is treated as one of the test items in the calibration
design, and its value as computed from each calibration run is the
basis for accepting or rejecting the calibration. All designs
cataloged in this Handbook have provision for a check standard.
The check standard should be of the same type and geometry as items that are measured in the designs. These artifacts must be stable and available to the calibration process on a continuing basis. There should be a check standard at each critical level of measurement. For example, for mass calibrations there should be check standards at the 1 kg; 100 g, 10 g, 1 g, 0.1 g levels, etc. For gage blocks, there should be check standards at all nominal lengths. A check standard can also be a mathematical construction, such as the computed difference between the calibrated values of two reference standards in a design. 

Database of check standard values 
The creation and maintenance of the database of check standard values
is an important aspect of the control process. The results from each
calibration run are recorded in the database. The best way to record
this information is in one file with one line (row in a spreadsheet) of
information in fixed fields for each calibration run. A list of
typical entries follows:
