As it turned out, the real problem was twofold: I had thought that the measurements would be thrown off most by not moving the protractor in a straight line, so I would end up with measurements that were too long. The most interesting part to me was that my bias was negative: my measurements were too short. You can enter a different number if you choose a different target. In Upper spec – lower spec, enter 4.5, for the diameter of a CD.In Reference, enter the known distance to the gummi bear.In Measurement Data, enter the column where you have your measurements.Choose Stat > Quality Tools > Gage Study > Type 1 Gage Study.Now we can use this data to perform a Type 1 Gage Study in Minitab Statistical Software: Most rulers and tape measures are very accurate in this day and age, so I’m going to choose a tool that makes the measurement process harder: a 6-inch protractor. 8.1 The Minitab worksheet Julies Gauge R&R Data 2 contains the data from the. This can seem a little tedious because the Type I Gage Study is evaluating a measurement tool. Exercise 1: Identifying Measurement Systems. Measure the location of the gummi bear 25 times. I'm using the diagonal distance of my computer monitor, 23 inches. Place the gummi bear at a spot where you know the distance from your starting point.Sadly, we still don’t need the catapult, and we only need one gummi bear.Choosing a target lets me provide specifications for the Type 1 Gage Study. I’ve mentioned that we will study the effect of different factors on how far a gummi bear flies, but the real goal of knowledge about factor effects is usually to minimize, maximize, or target a response variable. Later, we’ll look at the Gage Linearity and Bias Study and the Gage R&R Study. The Type 1 Gage Study is a starting point because it evaluates accuracy, precision, and consistency, but only for a single case. This involves measuring a single calibrated reference part a number of times, ideally 50 or. Today, we’ll look at doing a Type 1 Gage Study, which compares the measurement variation to the specifications for your process, to judge whether a gage is measuring well enough. The most fundamental MSA experiment is the Type 1 Gage Study. So at the end, I can have an acceptable Cgk in a measurement system that has bias (confirmed by the hypothesis test) and large variation in the measures (not repeatable) and conclude the opposite if I have specification limits wide enough to achieve a Cgk>1.33.Gummi bears have more to teach us about measurement systems analysis. In this type of study the part normally remains in situ. It involves repeated measurements of one part, on one instrument, by one operator to establish the baseline repeatability of the inspection process. A Cgk value of 1.33 is a common benchmark value to denote a capable gage – one that is both precise (good repeatability) and accurate (low bias). A Type 1 gage study is normally used to determine the baseline repeatability of a measurement system. Cgk decreases as the difference between the gage's mean measurement and the reference value increases. Cgk compares the Study Variation to the tolerance, but it also considers whether the measurements are "on target". repeatability tests (half of a Gage R&R study) on 1-2 parts, to see if there is any consistency. Minitab also calculates the capability metric Cgk to assess repeatability and bias together. A specific type of measurement system analysis (MSA). Basically because I can have a large variation in the measurement system (it is not repeatable) and at the same time obtain an aceptable Cg as soon as my specification limits are wide enough.Īfter this, Minitab mixes this Cg concept with bias in the Cgk: This Cg value indicates the gage's effectiveness within this tolerance range.įor me, this Cg seems an estimation of the capability of the measurement system, but not the repeatability. Values of Cg greater than 1.33 indicate the spread of the gage's measurements is adequately narrow in relation to your tolerance range.įor example, with the default values of K and L, a Cg metric of 2 indicates that 20% of your tolerance range will cover the entire spread of measurements twice over. To assess a gage's repeatability, Minitab calculates the Cg metric to compare the study variation (the spread of the gage's measurements) with a percentage of the tolerance. My confussion comes when using the specification limits and mixing them with a repeatability study-in the previous MSA I did, I didn't use this Type 1 Gage Study at Minitab.
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