What is Measurement System?

  • Posted on : December 31, 2009
  • Modified: November 5, 2020

  • Author : SPEC INDIA
  • Category : General

People measure product characteristics or process parameters so they can assess the performance of the system of interest. The measured values provide feedback of the process, so that people may adjust settings, replace tools, redesign fixtures or allow the operation to continue on its current course. The measurements are indeed the data that will allow people to make decisions critical to improvement efforts.

As critical as these measurements are, no measurement process is perfect. Sometimes different numbers or readings result when the same part or sample is measured a second time. Different readings may be made by different people, gauges or even by the same person using the same gauge. The difference in successive measurements of the same item is called measurement error. This source of variation must be analyzed, because the validity of the data directly affects the validity of process improvement decisions.

The measurement system is a major component of the process. In fact, studying variation within the parameters of the measurement system is of paramount importance for two reasons:

• Measurement error contributes to process variation and has a negative influence upon the process capability level.

• Measurement error is present whenever measurements are made. The effects of measurement error influence the assessment of all other items of the process.

In addition to being a part of the process or system, measurement activities may also form a process. It is totally inappropriate to view measurement error as merely a function of measurement hardware or instruments. Other components of the measurement process are equally important to measurement error or validity. For example, people contribute to measurement error by having different levels of tactile, auditory or visual perception. These characteristics account for calibration and/or interpretation differences.

Another example in measurement error is the contribution of a method change. This kind of error is one of the largest sources of variation in the measurement process. The significance of this is compounded when different people or instruments are used to evaluate the same item or process. Obviously, a standard procedure is needed for every measurement activity. Only this procedure should be used by all the people who operate test equipment. Measurement errors that are sometimes attributed to the different people collecting the data are actually due to differences in methodology. People are usually able to produce similar readings when they use the same methods for operating the measurement equipment. Other examples where measurement error may be introduced are: changes in environment, changes in test equipment, changes in standards and so on. In dealing with measurement error, one must be familiar with the concepts of true value, accuracy and precision.

True value. The true value is a theoretical number that is absolutely the “correct” description of the measured characteristic. This is the value that is estimated each time a measurement is made. The true value is never known in practice because of the measurement scale resolution, or the divisions that are used for the instrument’s output. For example, someone may be satisfied with a dimension of up to tenth of an inch (0.1). However, someone else may define that dimension with a different instrument up to the ten thousandth of an inch (0.1043), which, of course, is closer to the true value. The appropriate level of measurement resolution is determined by the characteristic specifications and economic considerations. A common practice and rule of thumb calls for tester resolution that is equal to or less than one-tenth of the characteristic tolerance (the upper specification limit minus the lower specification limit). The true value is considered as part of tester calibration and discussions of measurement accuracy.

Accuracy. The accuracy of a measurement system is the relationship between the average of a number of successive measurements of a single part and that of the true value. When the measurement process yields a mean of successive measurements that is different than the true value, the instrument is not calibrated properly.

Precision. The precision of a measurement system is quantified by the spread of readings that result from successive measurements of the same part or sample. The standard deviation of measurement error is used to quantify the spread of the precision distribution. The common variation that creates the precision distribution comes from two different sources:

• Repeatability (RPT). Repeatability is a measurement system variation that is due to a specific situation or set of conditions. The differences in successive measured readings for one item, made by one person, using one instrument, in one setting or environment, using one calibration of reference is error due to repeatability. This is the variation within a situation. Repeatability is present in every measurement system.

• Reproducibility (RPD). Reproducibility describes the difference in successive measurements for the same item that are due to differences of hardware, people, methods or environments. This source of variability is quantified as the spread (range or standard deviation) between several repeatability distributions—the distributions generated from repeated tries. Reproducibility exists only when there is more than one measurement situation.

Author: SPEC INDIA


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