In the context of measurement system analysis (MSA), trueness refers to the ability of a measuring instrument to deliver results that are as close as possible to the true value of the measured characteristic. In other words, it's a question of whether the measuring instrument delivers results while minimizing systematic errors.
To carry out a trueness study on a measurement method with quantitative data.
- Click on the Gage R&R menu
- In the horizontal bar: click on the "Accuracy" submenu
- Click on "add a precision
- Complete the information on the measurement method to be validated.
⇒ Name Designation of the quantitative characteristic measured, e.g. diameter, force, pressure, etc.
⇒ ProductsNumber of products.
⇒ True value Expected value of the standard.
⇒ The value of k Number of measurement repetitions per operator per part.
⇒ K valueValue used to calculate capability indicators Cg, Cgk . the k values proposed in Ellistat data analysis come from two standards.
- k=3 which comes from the AIAG reference
- k=2 which comes from the VDA reference
the term 2*k corresponds to the number of standard deviations used to calculate the indicators Cg, Cgk.
This usually corresponds to 6 (the 6 here simply corresponds to k=3 -according to AIAG-). If the sample comes from a normally distributed population, then approximately 99.73% of all data points would fall within this range. Other commonly used values are 5.15 for a coverage of 99% and 4 (the 4 here simply corresponds to a k=2-according to VDA-) for a coverage of 95%, respectively.
⇒ Reference: The name of the control method equipment or control machine.
⇒ Minimum and maximum tolerancesin the following example 9.5mm≤ Diameter ≤10.5mm , maximum tolerance is 10.5 and minimum tolerance is 9.5.
⇒ Production standard deviation : Set the historical standard deviation of the measured characteristic. This standard deviation is used to calculate the number of distinct categories.
- Pressing Validate opens the Justesse page below. This page has two zones.
⇒ Zone 1This contains all the information entered in the previous step. Such as the number of parts, the true value, the number of repetitions, the calculation method, or the k value.
⇒ Zone 2 This is where you enter the results obtained by the same operator.
2. Using the results of the trueness study :
The result is displayed in the center of zone 2. In this window, several tables and graphs are displayed:
⇒ Statistical resultsThis table presents statistical results such as number of measurements, resolution of measuring equipment, repeatability, mean bias, Cg, Cgk and other indicators.
A T-Test report that compares the mean value of the measurements with the expected value to check the significance of the difference between the mean of the measurements and the expected value.
⇒ Moustache box and control chart graphicsThese graphs show the dispersion of the data and the position in relation to the expected value.
The video below shows the various steps involved in carrying out a trueness study.