Tendency

Tendency (or deviation) is the difference between observed average of measurement and reference value. Reference value, also known as accepted reference value or standard value, is a value that can be used as agreed reference to measured values. a reference value can be determined by the average of a series of measurement procedures performed with a measurement equipment of greater accuracy.

Example:

Metrological laboratory or tool shop equipment.

Tendency is often named by the usage of accuracy term. Once accuracy has various meanings in the literature, it is not recommended its use as an alternative to tendency.

To determine measurement system tendency in a specific position of process amplitude, it is necessary to obtain a standard value accepted to a part. This can be done with inspection equipment of metrology or tool shop. A reference value is obtained from these reading to be compared with the average results of the operators, identified as calculated in the R&R measurement device study.

When all parts of this sample cannot be measured this way, the alternative method can be used:

1) Measure one of these sample parts with accuracy, as inspection equipment in the metrology laboratory;

2) Ask to an operator to measure the same part at least 10 times, using a measurement device that has been evaluated.

3) Calculate the readings average. The difference between reference value and observed average represents the measurement system tendency (see also Linearity section).

If a index is desired, convert tendency in a process variation percentage (or tolerance), multiplying by 100 and dividing by process variation (or tolerance).

If tendency id relatively big, search for these possible causes:

  
A measurement system tendency is obtained as follows:

where:

= Observed Average

VR = Reference Value

, indicating the predominance of tendency on the Process variation, where VP is process variation

M.S.A. Graphic by Stability

To generate a M.S.A. graphic by Tendency, perform a M.S.A Graphic Query. and select in the parameter Type the option Tendency.