Measurement

Custom Search

Get Free Six Sigma Tools Here

Purpose

Measurement is used to record data against a criteria (or standard) for the purpose of gaining information and knowledge about a process or feature.


Anatomy

Measurement

Measurement

Terminology

A. Comparison between the entity and the standard

B. Count of strokes

C. Referring to a standard unit (meter or inch)

D. Possible errors in the measurement

Reliability


Purpose

To explain the reliability of measurement instruments. Any error in these instruments influences the ability to judge conformance. A measuring instrument is said to be reliable when it yields the same results on repeated trials. In other words, a reliable instrument has negligible measurement error because it is more accurate and precise.

Anatomy

Measurement Reliability

Accuracy


Reference: Juran’s Quality Control Handbook

Terminology

A. Accuracy: After making numerous repeated measurements on a single product characteristic with a measuring instrument, the extent to which the computed average of these measurements agrees with the “true” value of the characteristic reflects the “accuracy” of the measurement system.

The difference between the average and “true” value is also called the “error”, also known as “systemic error” or “bias”. An accuracy problem generally indicates that the measuring instrument needs to be calibrated.

Precision: After making numerous repeated measurements on a single product characteristic with a measuring instrument, the extent to which the instrument is able to reproduce it’s own measurements is called “precision”.

Regardless of accuracy of calibration, experience shows that a measuring instrument will not give identical measurement readings, even when making a series of measurements on the same product characteristic.

Precision generally describes the inherent variation of a specific measuring instrument and is usually evaluated by the metrology function.

Linearity


Purpose

To explain the reliability of a measuring instrument by determining if there is linearity error. There is a change in the accuracy of the measuring instrument throughout the expected operating range of the gage.

Anatomy

Linearity

Linearity

Terminology

A. True or reference value

B. Observed Average Value

C. Lower part of the range

D. Higher part of the range

E. Linearity

Linearity is the change in accuracy through the expected operating range of a measuring instrument. A linearity error may result from a calibration error, wear of the measuring instrument. etc. or simply this type of error is natural for the instrument.

Repeatability


Purpose

To study the measurement system that is used to collect data. One important source of error associated with a measurement system is repeatability, which describes the variation obtained in measurements made by one instrument over repeated trials.

Anatomy

Repeatability

Repeatability

Reference: Measurement Systems Analysis (Chrysler, Ford, GM)

Terminology

A. Repeatability:

Repeatability is the variation in measurements obtained when an operator measures the same characteristic several times with the same instrument under the following conditions:

  • on the same part;
  • in the same location on the part;
  • under the same condition of use;
  • over a short period of time.

Repeatability is considered to be the inherent variation of the measurement system. The repeatability error comes from the instrument itself and the position of the part in relation to the instrument.

Note that measurements may show good repeatability without being accurate. In contrast to precision, where the measuring instrument is generally evaluated by the metrology function, repeatability is evaluated through a gage R&R study of the measuring system.

Reproducibility


Purpose

To study the measurement system that is used to collect data. One important source of error associated with a measurement system is reproducibility. This describes the variation introduced by different operators in the measurement process.

Anatomy

Reproducibility

Reproducibility

Reference: Measurement Systems Analysis (Chrysler, Ford, GM)

Terminology

A. Reproducibility

Reproducibility is the variation between the average of the measurements obtained by different operators who measure the same characteristic on the same parts under the following conditions:

  • the same measuring instrument;
  • the same method;
  • in the same location on the parts;
  • under the same conditions of use;
  • over a short period of time.

Reproducibility represents the incremental bias that can be attributed to each operator. If there is variability between the operators, then the reproducibility of the measurements represents the variation between operators. Reproducibility can be evaluated through a gage R&R study.

Stability


Purpose

To describe the total variation in the measurement system over time. If there is a change in the measurements over an extended period of time, there is a problem related to the stability of the measurements.

Anatomy

Stability

Stability

Terminology

A. The distribution of measurements at time 1.

B. The distribution of measurements at time 2.

C. Stability

Stability is the variation in the measurements obtained with a measurement system on the same master or parts when measuring a single characteristic over an extended time period. Measurement stability may vary according to instrument operating conditions, wear, handling, maintenance, etc.

From Measurement to Free Six Sigma.

Recent Articles

  1. Quality Control Plan - Simple Method To Create Effective Plans

    May 10, 16 09:24 PM

    A Quality Control Plan is a documented description of the activities needed to control a process or product. The objective of a QCP is to minimize variation.

    Read More

  2. Get Free Six Sigma Tools

    May 10, 16 08:49 PM

    The Largest Collection of Free Six Sigma Tools and Training on the Web!

    Read More

  3. Weibull Distribution

    May 10, 16 07:28 PM

    The Weibull distribution is applicable to make population predictions around a wide variety of patterns of variation.

    Read More