【正文】 or measures each unit repeatedly ? Data must be balanced for Minitab—each operator must measure each unit the same number of times ? The units should represent the range of variation in the process ? Operators should randomly and ―blindly‖ test the units。R = m / t If the process variation is small pared to the tolerance spread, then m may be pared a ―dummy‖ process spread : %P/T = m / (tolerance / ) ?Alternatively, Minitab shows the number of distinct categories the measurement system can distinguish : * p / m 17 Gage Ramp。R Review 4 List Common Problems With Measurements You Deal With ? ? ? ? ? ? ? 5 Common Problems with Measurements Problems with the measurements themselves 1. Bias or inaccuracy: The measurements have a different average value than a ―standard‖ method. 2. Imprecision: Repeated readings on the same material vary too much in relation to current process variation. 3. Not reproducible: The measurement process is different for different operators, or measuring devices or labs. This may be either a difference in bias or precision. 4. Unstable measurement system over time: Either the bias or the precision changes over time. 5. Lack of resolution: The measurement process cannot measure to precise enough units to capture current product variation. 6 Desired Measurement Characteristic for Continuous Variables Good accuracy if difference is small Standard value Observed value Data from repeated measurement of same item Good repeatability if variation is small * 1. Accuracy The measured value has little deviation from the actual value. Accuracy is usually tested by paring an average of repeated measurements to a known standard value for that unit. 2. Repeatability The same person taking a measurement on the same unit gets the same result. 7 Desired Measurement Characteristic for Continuous Variables, cont. 3. Reproducibility Other people (or other instruments or labs) get the same result you get when measuring the same item or characteristic. * Small relative to a) product variation and b) product tolerance (the width of the product specifications) D ata from Pa rt XD ata Co llector 1D ata Co llector 2Good reprodu cib il ity if diffe re nce is sm all *D ata from Pa rt X8 Desired Measurement Characteristic for Continuous Variables, cont. 4. Stability Measurements taken by a single person in the same way vary little over time. * Small relative to a) product variation and b) product tolerance (the width of the product specifications) * 9 Desired Measurement Characteristic for Continuous Variables, cont. 5. Adequate Resolution There is enough resolution in the measurement device so that the product can have many different values. 5 .1 5 . 2 5 . 3 5 . 4 5 . 5X XXXXXXXXXXXXXXXGood if 5 or more distinct values are observed 10 Improving a Measurement System A measurement system consists of ? Measuring devices ? Procedures ? Definitions ? People To improve a measurement system, you need to ?Evaluate how well it works now