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:

1. Control of instrument precision or short-term variability

2. Control of bias and long-term variability

• Example of a Shewhart control chart

• Example of an EWMA control chart

The short-term 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

• n = number of difference measurements

• m = number of artifacts.

Measurements on a check standard provide the mechanism for controlling the bias and long-term 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.

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:

1. Date

2. Identification for check standard

3. Identification for the calibration design

4. Identification for the instrument

5. Check standard value

6. Repeatability standard deviation from design

7. Degrees of freedom

8. Operator identification

9. Flag for out-of-control signal

10. Environmental readings (if pertinent)