2. Measurement Process Characterization
2.2. Statistical control of a measurement process
2.2.3. How is short-term variability controlled?

2.2.3.2.

Data collection

Case study: Resistivity

A schedule should be set up for making measurements with a single instrument (once a day, twice a week, or whatever is appropriate for sampling all conditions of measurement).

Short-term standard deviations

The measurements are denoted

where there are J measurements on each of K occasions. The average for the kth occasion is:

The short-term (repeatability) standard deviation for the kth occasion is:

with (J-1) degrees of freedom.

Pooled standard deviation

The repeatability standard deviations are pooled over the K occasions to obtain an estimate with K(J - 1) degrees of freedom of the level-1 standard deviation

Note: The same notation is used for the repeatability standard deviation whether it is based on one set of measurements or pooled over several sets.

Database

The individual short-term standard deviations along with identifications for all significant factors are recorded in a file. The best way to record this information is by using one file with one line (row in a spreadsheet) of information in fixed fields for each group. A list of typical entries follows.

1. Identification of test item or check standard
2. Date
3. Short-term standard deviation
4. Degrees of freedom
5. Instrument
6. Operator